(CNN) There is still time for Puerto Ricans to get to a government-run shelter before powerful Hurricane Maria clobbers the island, Gov. Ricardo Rosselló told CNN late Tuesday.
But the window of opportunity is closing.
"There's still time ... but people need to move fast," Ricardo Rosselló told CNN's "Anderson Cooper 360˚," calling the storm the "worst hurricane in modern history in Puerto Rico."
US President Trump tweeted his thoughts to those on the island, a US territory. "Be careful, our hearts are with you," he wrote, calling the storm a "monster hurricane," and pledging assistance.
Maria is a Category 5 storm with sustained winds of 175 mph (281 kph). By late Tuesday, the eye of hurricane was expected to move near or over St. Croix in the US Virgin Islands and make landfall in Puerto Rico Wednesday morning.
"Winds are starting to pick up, the glass is starting to bow back and forth. We're soon going to take shelter in a safe room," Mike Theiss, a hurricane chaser who is staying in Fajardo, told CNN's Don Lemon.
Maria has already obliterated parts of Dominica and killed at least one person in Guadeloupe.
Rosselló said there are about 500 shelters open. Emergency workers are still out, but will head in when sustained winds reach 50 mph, the governor said.
Many people live on the eastern half of Puerto Rico, which forecasters worry will endure the brunt of the most powerful hurricane ever to hit the island.
Residents of Puerto Rico, who were spared some of Irma's wrath when that hurricane's core passed north of the island, boarded up homes and businesses Tuesday. People who fled to the US territory as Maria, and before it, Hurricane Irma, which swept through the region just days ago, advanced on their islands sought refuge in hotels and shelters.
Hurricane Maria slams the Caribbean
An apartment building is missing a wall in San Juan, Puerto Rico, on Monday, September 25, nearly a week after Hurricane Maria devastated the US commonwealth. Power is still out in most places, and communications remain almost nonexistent on the island of 3.4 million people.
Yancy Leon rests at the Luis Muñoz Marin International Airport near San Juan on September 25. She's been waiting in line for two days to get a flight out.
An aerial view shows the flooding in San Juan on September 25.
People collect water from a natural spring created by landslides in Corozal, Puerto Rico, on Sunday, September 24. Puerto Rican Gov. Ricardo Rosselló said the island faces a humanitarian crisis.
An aerial view shows a flooded neighborhood in Catano, Puerto Rico, on Friday, September 22.
A man cleans a muddy street in Toa Baja, Puerto Rico, on September 22.
A man walks on a highway divider while carrying his bicycle through San Juan, Puerto Rico, on Thursday, September 21.
A shack is destroyed in San Juan on September 21.
A gas station's sign is damaged in Punta Cana, Dominican Republic, as the hurricane passed just north of the country on September 21.
Rescue workers drive through a flooded road in Humacao, Puerto Rico, on Wednesday, September 20.
A mattress that fell from the third floor is surrounded by debris outside a San Juan apartment complex on September 20.
Damage is seen in Roseau, Dominica, on September 20.
People walk through the destruction in Roseau on September 20.
San Juan is shrouded in darkness after the hurricane knocked out power to the entire island of Puerto Rico.
Power lines are scattered across a road in Humacao, Puerto Rico, on September 20.
Residents move aluminum panels from an intersection in Humacao on September 20.
Rescue vehicles are trapped under an awning in Humacao on September 20.
Trees are toppled outside the Roberto Clemente Coliseum in San Juan on September 20.
Members of a rescue team embrace as they wait to help in Humacao on September 20.
A tree is damaged in Fajardo, Puerto Rico, on September 20.
Debris is strewn across a Fajardo street on September 20.
A woman closes her property in Naguabo, Puerto Rico, hours before Maria's arrival.
People take shelter at Puerto Rico's Humacao Arena on Tuesday, September 19.
Two girls play on cots at the Humacao Arena.
Waves crash in San Juan as the hurricane neared Puerto Rico on September 19.
People pray in Humacao on September 19.
A street is flooded in Pointe-a-Pitre, on the French Caribbean island of Guadeloupe, on September 19.
People stand near debris at a restaurant in Le Carbet, Martinique, on September 19.
People in Luquillo, Puerto Rico, board up windows of a business on September 19.
A boat is overturned off the shore of Sainte-Anne, Guadeloupe, on September 19.
Cars line up at a gas station in San Juan on September 19.
A motorist drives on the flooded waterfront in Fort-de-France, Martinique, on September 19.
Floodwaters surround cars in Pointe-a-Pitre on September 19.
Soldiers patrol a street in Marigot, St. Martin, as preparations were made for Maria on September 19.
People buy provisions in Petit-Bourg, Guadeloupe, as the hurricane approached on Monday, September 18.
Customers wait in line for power generators at a store in San Juan on September 18.
Fraught journey
Jessica Mangiaracina and Bob Perkins came to San Juan from St. Thomas with their two children. Irma destroyed everything around them and threw the island into chaos.
It took them six days to get off St. Thomas, they said, killing chickens for food and making their way through areas being looted.
They got to Puerto Rico by catamaran, where, like many others, they tried to get a flight out with no success.
Now they are coping with the reality that they've escaped into the middle of Hurricane Maria's angry path.
The couple is among thousands of people in shelters.
When asked what their current plan is, Perkins responded, "survive."
Heather Farrell, a visitor to the island, is on her honeymoon with her husband Luke. They were married on September 9. She says that they had tried to cut their trip short when it became apparent that they were in Maria's firing line.
"We did try to get off, as early as Saturday but all flights were either booked or canceled. We actually are on the ocean -- our room faces the ocean. It's pretty windy but there is no rain. We'll stay inside for now."
She said that hotel staff had asked that all guests that are staying at the hotel come downstairs early Wednesday morning to a safe room that they have set up for them.
"I would rather be home than here but I guess we're making the best of it," she said.
Worst storm in almost 80 years
Millions of Puerto Ricans are hoping to survive what could be the most powerful storm to ever hit the island. Maria likely will be the first Category 4 or 5 hurricane to make landfall in Puerto Rico in more than eight decades.
"No generation has seen a hurricane like this since San Felipe II in 1928," Rosselló said earlier. "This is an unprecedented atmospheric system."
President Donald Trump said the federal government will aid Puerto Rico in the aftermath of the storm.
"Puerto Rico being hit hard by new monster Hurricane," the President tweeted. "Be careful, our hearts are with you -- will be there to help!"
Maria has already killed one man in Guadeloupe after he ignored orders to stay inside and was struck by a falling tree, the island's government said.
Track the storm
Man killed by falling tree in Guadeloupe
Two other people are missing after a boat sank off the coast of La Désirade, a smaller island near the mainland of Guadeloupe. The government said about 80,000 people, or 40% of the households on the island, are without power.
Hurricane Maria caused widespread flooding Tuesday in Point-A-Pitre, Guadeloupe.
The storm also caused "widespread devastation" in Dominica, Prime Minister Roosevelt Skerrit said Tuesday.
The hurricane shredded the prime minister's house overnight and left much of the island -- population 73,000 -- in ruins.
Overseas, relatives were trying to reestablish contact with loved ones on the island.
Petrona Joseph, a public relations consultant who lives in Montreal, told CNN that her sister in Dominica was at home in Roseau, huddling in the bathroom with the rest of the family when they lost their roof.
They thought about trying to go to a neighbor's home but the winds were too strong. Joseph last heard from her sister by text Monday night around 11 p.m.
Maria is now the strongest hurricane on record to make landfall in Dominica, a former French and British colony whose economy relies heavily on tourism and agriculture.
Puerto Rico says Maria 'will be catastrophic'
Maria could bring up to 2 feet of rain and a storm surge of up to 9 feet to Puerto Rico, sparking flooding and mudslides, CNN Meteorologist Allison Chinchar said.
In the capital city of San Juan, residents cleared store shelves of water and other supplies.
A CNN producer who was driving from the airport Tuesday said there were still some vehicles on the road. Of the two gas stations she passed, one was open. Many shops were already closed, she observed.
Some tourists from California told her that the hostel they are staying at moved them to an interior room when the men were unable to find flights home.
Gov. Rosselló has declared a state of emergency, and Trump has issued an emergency declaration for Puerto Rico to aid with federal assistance.
Puerto Rico sheltered many of the evacuees who fled from other Caribbean Islands during Hurricane Irma earlier this month. Now those evacuees and native Puerto Ricans are bracing for devastation.
"This is an event that will be damaging to the infrastructure, that will be catastrophic," Rosselló said. "Our only focus right now should be to make sure we save lives."
"We expect to feel storm winds, tropical storm winds, (from) Tuesday up until late on Thursday. That's about two-and-a-half days of tropical storm winds," Rosselló said.
The Puerto Rico Convention Center in the capital San Juan to the north -- which is still housing Hurricane Irma evacuees from other Caribbean islands -- is preparing to accept thousands of residents as the worst of the storm is felt.
Martinique largely spared
One bit of good news emerged from the Caribbean: The French island of Martinique suffered no major damage, the French Interior Ministry tweeted Tuesday.
Maria knocked out power to about 50,000 homes, and 10,000 homes had no water. But overall, the damage assessment was "reassuring," the French Interior Ministry said.
The director general of French civil security, Jacques Witkowski, said only two people on Martinique suffered minor injuries.
Hurricane Irma tears through Caribbean
Carmelo Mota, a builder, searches for tools in his destroyed home in Charlotte Amalie, US Virgin Islands, on Monday, September 18. Hurricane Irma devastated the US territory and other Caribbean islands in the region, leaving them exposed to new storms brewing in the Atlantic.
An aerial photo shows the devastation in Road Town, the capital of Tortola, the largest and most populated of the British Virgin Islands, on Wednesday, September 13.
UK Foreign Secretary Boris Johnson talks to a resident of Anguilla during a visit on September 13.
People collect food that was delivered by emergency workers in the Sandy Ground area of Marigot, St. Martin, on Tuesday, September 12.
Buildings are destroyed in St. Martin on September 12.
French President Emmanuel Macron shakes hands with St. Martin residents during a visit to the island on September 12.
French soldiers patrol St. Martin on September 12.
A person works to clean up a street September 12 after Hurricane Irma flooded parts of Havana, Cuba.
A man makes repairs in Havana on September 12.
This Marigot church was among the buildings destroyed in the storm.
Cubans affected by Hurricane Irma line up to collect drinking water in Isabela de Sagua, Cuba, on Monday, September 11.
Dutch King Willem-Alexander, front right, tours damage in St. Maarten on September 11.
A palm tree sticks out of a pool on the French side of St. Martin on September 11.
A woman stands next to her water-logged belongings that had been laid out to dry in front of her home in Isabela de Sagua on September 11.
People line up for supplies in St. Martin on September 11.
The skeleton of a boat drifts in St. Martin's Simpson Bay on September 11.
People salvage material from the remains of a house in Isabela de Sagua on September 11.
Members of the British Army provide support on Tortola, one of the British Virgin Islands.
A woman carries a dog at an airport checkpoint in St. Martin on September 11.
People wade through a flooded street as a wave crashes in Havana on Sunday, September 10.
Two men search through the rubble of their St. Martin restaurant on September 10.
People make their way through debris in the Cojimar neighborhood of Havana on September 10.
People board a plane leaving St. Martin on September 10.
A man wades through a flooded street in Havana on September 10.
An overview of Havana shows flooded streets on Saturday, September 9.
A woman surveys flooding in Havana on September 9.
A boat rests in a cemetery after Irma tore through Marigot, St. Martin.
Residents return home after Irma passed through Caibarien, Cuba, on September 9.
A man walks in Caibarien on September 9.
A man carries a child through a flooded street in Fort-Liberte, Haiti, on Friday, September 8.
A man walks on a St. Martin street covered in debris on September 8.
A damaged home is tilted onto its side on the Puerto Rican island of Culebra on Thursday, September 7.
A home is surrounded by debris in Nagua, Dominican Republic, on September 7.
Irma damage is seen in St. Martin's Orient Bay on September 7.
Employees from an electrical company work to clear a fallen tree in Sanchez, Dominican Republic.
A woman makes her way through debris in Fajardo, Puerto Rico, on September 7.
In this image made from video, damaged houses are seen in St. Thomas on September 7.
The storm left widespread destruction on the island of Barbuda on September 7.
A flattened home is seen in Nagua, Dominican Republic, on September 7.
Nagua residents ride through an area affected by the storm on September 7.
Trash and debris is washed ashore in Cap-Haitien, Haiti, on September 7.
People walk through damage in Marigot, St. Martin, on September 7.
People survey damage in Marigot on September 7.
Bluebeard's Castle, a resort in St. Thomas, was hit hard by Irma. St. Thomas resident David Velez sent this photo to CNN on September 7.
Irma ruined these vehicles in St. Thomas.
Waves smash into St. Martin on Wednesday, September 6.
A man looks at a vehicle turned upside down in the British territory of Anguilla.
An aerial view of St. Martin on September 6.
Damaged cars are seen on a St. Martin beach on September 6.
A boat is washed onto shore in St. Martin.
Cars are piled up in Marigot on September 6.
A man walks past damaged buildings in St. Martin on September 6.
A car is flipped onto its side in Marigot.
Broken palm trees are scattered on a Marigot beach on September 6.
Irma floods a beach in Marigot on September 6.
Rapid intensification
In just 30 hours, Maria's intensity exploded from 65 mph on Sunday to 160 mph by Monday night, the National Hurricane Center said.
The British Foreign Office said more than 1,300 troops are on standby, either on affected islands or in nearby locations, ready to help after Maria tears through.
One military team has been deployed to the British Virgin Islands, and a British military reconnaissance team is on standby to go to the British territory of Montserrat.
The HMS Ocean is set to arrive in the area at week's end with 60 tons of government supplies.
CNN's Jaide Timm-Garcia, Sarah Faidell, Matou Diop, Jamie Gray, Judson Jones, Brandon Miller, Taylor Ward and Karen Smith contributed to this report.