(CNN) Two more earthquakes shook southern Mexico on Saturday, further rattling a country still coming to grips with the devastation from stronger temblors earlier this month.
A 6.1 magnitude earthquake Saturday morning was centered in Oaxaca state near Matias Romero, a town about 275 miles southeast of Mexico City, the US Geological Survey said. Roughly speaking, the epicenter was between the centers of this month's two more violent earthquakes -- the 7.1 magnitude temblor that hit Tuesday closer to the capital, and the 8.1 magnitude quake that struck September 8 off the southern Pacific coast, near Chiapas state.
People evacuate buildings in Mexico City's Tlatelolco area after a seismic alert sounded Saturday.
A 4.5 magnitude quake hit Oaxaca at 7:06 p.m. ET. That temblor occurred at a depth of 8.9 kilometers, according to initial readings by USGS.
In Oaxaca, some highways and a bridge that had been damaged during the September 8 earthquake collapsed, Mexico's federal police said.
Mexico City did not appear to have sustained significant damage in the earlier and stronger of Saturday's two quakes, said the country's office of the secretary of public security.
Warning sirens sounded in Mexico City after the morning quake was detected, interrupting rescue operations at some of the dozens of buildings that collapsed from Tuesday's earthquake.
CNN video showed rescuers walking off one vast pile of rubble to more stable ground in case any shaking shifted debris further.
Mexico has had three earthquakes greater than 6.0 magnitude since September 8.
A 6.1 magnitude quake can produce strong shaking and considerable damage to poorly built structures and slight to moderate damage in better-constructed buildings, the USGS says.
The earlier major quakes killed hundreds of people and turned buildings into dust and debris in parts of Mexico. More than 300 people have been reported killed in Tuesday's quake; nearly 100 reportedly died in the September 8 temblor.
Searches continue for survivors
Search and rescue efforts in the Mexico City area could last "for at least two more weeks," Luis Felipe Puente, Mexico's civil protection coordinator, told CNN affiliate Foro TV..
Deadly earthquake rocks central Mexico
Relatives of missing people wait for news in front of a collapsed building in Mexico City on Friday, September 22. A magnitude 7.1 quake
hit central Mexico three days earlier.
Rescue workers search for survivors Thursday, September 21, at a collapsed apartment building in Mexico City.
A religious statue is salvaged from a former convent that was heavily damaged in Tlayacapan, Mexico. This was the second earthquake to hit Mexico in two weeks. A magnitude 8.1 quake
struck off the country's southern coast on September 8.
A wall is damaged at a home in Tlayacapan on Wednesday, September 20.
Volunteers organize donations in Mexico City on September 20.
Rescuers in Mexico City work to save a child
trapped inside the Enrique Rebsamen elementary school on September 20. Rescue workers said they believed they'd made contact with a girl trapped in the rubble at the school. But by the next afternoon, navy official Angel Enrique Sarmiento said all the school's children had been accounted for and there was no student in the rubble.
He apologized for the confusion.
Men carry beams of wood to offer help in Mexico City's Roma neighborhood on September 20.
Families prepare to sleep under tarps outside their quake-damaged building in Mexico City on September 20.
Rescuers and firefighters lower a corpse from a house in Mexico City on September 20.
Volunteers and rescue workers search for people trapped inside the Enrique Rebsamen school on September 20.
A volunteer in Mexico City asks for silence as a flattened building is searched for survivors on September 20.
Members of the Mexican Army nap September 20 after assisting in search-and-rescue missions in Mexico City.
People look for survivors in Mexico City on September 20.
A man walks his bike past a partially collapsed building in Jojutla on September 20.
Volunteers arrange food and other donated supplies at a distribution point in Mexico City on September 20.
Children's toys are seen in a damaged building in Mexico City on September 20.
A survivor is pulled out of rubble in Mexico City on September 20.
People sleep on the street next to damaged homes in Jojutla on September 20.
Soldiers remove debris from a collapsed building in Mexico City on September 20.
An injured person is carried away after being rescued in Mexico City on Tuesday, September 19. The earthquake happened on the anniversary of a 1985 quake that killed an estimated 9,500 people in and around Mexico City.
Rescue workers remove rubble from a Mexico City building on September 19.
Rescue workers in Mexico City search for people trapped inside the collapsed Enrique Rebsamen school on September 19.
A man comforts a student outside a school in Mexico City on September 19.
A police officer runs toward the site where a building collapsed in Mexico City on September 19.
Rescue workers and volunteers search a collapsed building in Mexico City on September 19.
A man is rescued under rubble in Mexico City's Condesa area on September 19.
A woman's crushed body hangs from a collapsed building in Mexico City on September 19.
Recovery efforts take place at the collapse of a residential building in Mexico City on September 19.
The quake damaged the Jojutla Municipal Palace.
A search goes on at the scene of a collapsed building in Mexico City's Del Valle neighborhood on September 19.
Emergency workers remove debris as they search for survivors in Mexico City on September 19.
People react in Mexico City just after the quake hit.
People gather on a Mexico City street after office buildings were evacuated because of the quake.
A building is damaged in Mexico City on September 19.
A woman in Mexico City cries as she tries to reach people on her cell phone after the quake.
Patients from a Mexico City hospital receive treatment outside after the hospital was evacuated on September 19.
A woman in Mexico City reacts after the quake.
People remove debris off a building that collapsed in Mexico City.
A car is crushed by debris in Mexico City on September 19.
A woman receives medical assistance after she was injured in Mexico City on September 19.
People stand inside a Mexico City building that collapsed in the quake.
"Our first phase is rescue and humanitarian aid," Puente said. "Until we are absolutely certain that there are no more people missing, we will continue our search and rescue mission."
How to help earthquake victims in Mexico
An unaccounted number of people went to shelters around the capital after losing their homes. Schools closed indefinitely, and millions initially were without power.
A hospitalized survivor spoke to CNN about his ordeal after being trapped in rubble for 17 hours.
Martin Mendez, a locksmith, said he was working in a building when it collapsed. He said he was overwhelmed by the pain of a broken bone and worried his life-sustaining air would run out.
Three other people, he said, tumbled around or on him. One woman fell with such force his right leg was broken.
"We had to hold on," he said. The four calmed each other as the hours passed.
Rescuers would yell, "Is anyone in there alive?" but they couldn't hear the group's frantic replies.
Diana Pacheco texted her husband, told him where they were. Still no rescue.
Mendez said they feared the air would run out. But he kept his faith.
"I always believed I was going to get out alive," he said.
On Wednesday morning, they were pulled out.
CNN's Ed Lavandera, Rosa Flores, Steve Almasy, Artemis Moshtaghian and Nicole Chavez contributed to this report.