(CNN) Libyan forces have retaken parts of Sirte from ISIS militants, gaining ground in the extremist group's most significant stronghold outside Syria and Iraq, according to a Libyan military group.
ISIS on Europe's doorstep
However, Libyan forces encountered fierce resistance Sunday, which included three suicide car bombs. One detonated near a field hospital in the city, according to the media wing of Al-Bunyan al-Marsous, a military offensive led by Libyan forces from Misrata.
The ISIS terror threat
People flee the scene of a terror attack at Istanbul's Ataturk airport on June 29. Turkish officials have strong evidence that ISIS leadership was involved in the planning of the attack, a senior government source told CNN. Officials believe the men -- identified by state media as being from Russia, Uzbekistan and Kyrgyzstan -- entered Turkey from the ISIS stronghold of Raqqa in Syria, bringing with them the suicide vests and bombs used in the attack,
the source said.
The ISIS militant group -- led by Abu Bakr al-Baghdadi, pictured -- began as a splinter group of al Qaeda.
Its aim is to create an Islamic state, or caliphate, across Iraq and Syria. It is implementing Sharia law, rooted in eighth-century Islam, to establish a society that mirrors the region's ancient past. It is known for killing dozens of people at a time and carrying out public executions.
Kurdish Peshmerga fighters fire missiles during clashes with ISIS in Jalawla, Iraq, on June 14, 2014. That month, ISIS took control of Mosul and Tikrit, two major cities in northern Iraq.
Traffic from Mosul lines up at a checkpoint in Kalak, Iraq, on June 14, 2014. Thousands of people
fled Mosul after it was overrun by ISIS.
ISIS fighters parade down an Iraqi street in this image released by the group in July 2014.
Aziza Hamid, a 15-year-old Iraqi girl, cries for her father while she and other Yazidi people are flown to safety after a
dramatic rescue operation at Iraq's Mount Sinjar on August 11, 2014. A CNN crew
was on the flight, which took diapers, milk, water and food to the site where as many as 70,000 people were trapped by ISIS. Only a few of them were able to fly back on the helicopter with the Iraqi Air Force and Kurdish Peshmerga fighters.
On August 19, 2014, American journalist James Foley
was decapitated by ISIS militants in a video posted on YouTube. A month later, they released videos showing the executions of American journalist Steven Sotloff and British aid worker David Haines.
ISIS militants stand near the site of an airstrike near the Turkey-Syria border on October 23, 2014. The United States and several Arab nations
began bombing ISIS targets in Syria to take out the group's ability to command, train and resupply its fighters.
A Kurdish marksman stands atop a building as he looks at the destroyed Syrian town of Kobani on January 30, 2015. After four months of fighting, Peshmerga forces
liberated the city from the grip of ISIS.
Safi al-Kasasbeh, right, receives condolences from tribal leaders at his home village near Karak, Jordan, on February 4, 2015. Al-Kasasbeh's son,
Jordanian pilot Moath al-Kasasbeh, was burned alive in a video that was released by ISIS militants. Jordan is one of a handful of Middle Eastern nations taking part in the U.S.-led military coalition against ISIS.
In February 2015, British newspapers report the identity of "Jihadi John," the disguised man with a British accent who had appeared in ISIS videos executing Western hostages. The militant was identified as Mohammed Emwazi, a Kuwaiti-born Londoner. On November 12, 2015, the Pentagon announced that Emwazi was in a vehicle
hit by a drone strike. ISIS later confirmed his death.
In March 2015, ISIS released video and images of a man being thrown off a rooftop in Raqqa, Syria. In the last photograph, the man is seen face down, surrounded by a small crowd of men carrying weapons and rocks. The caption reads "stoned to death." The victim was brutally killed
because he was accused of being gay.
An Iraqi soldier searches for ISIS fighters in Tikrit on March 30, 2015. Iraqi forces
retook the city after it had been in ISIS control since June 2014.
Dead bodies lie near a beachside hotel in Sousse, Tunisia, after
a gunman opened fire on June 26, 2015. ISIS claimed responsibility for the attack, which killed at least 38 people and wounded at least 36 others, many of them Western tourists. Two U.S. officials said they believed the attack might have been inspired by ISIS but not directed by it.
ISIS also claimed responsibility for what it called a suicide bombing
at the Al-Sadiq mosque in Kuwait City on June 26, 2015. At least 27 people were killed and at least 227 were wounded, state media reported at the time. The bombing came on the same day as the attack on the Tunisian beach.
A man inspects the aftermath of a car bombing in Khan Bani Saad, Iraq, on July 18, 2015.
A suicide bomber with an ice truck, promising cheap relief from the scorching summer heat, lured more than 100 people to their deaths. ISIS claimed responsibility on Twitter.
Two women hold hands after an explosion in Suruc, Turkey, on July 20, 2015. The blast
occurred at the Amara Cultural Park, where a group was calling for help to rebuild the Syrian city of Kobani, CNN Turk reported. At least 32 people were killed and at least 100 were wounded in the bombing. Turkish authorities said they believed ISIS was involved in the explosion.
Spectators at the Stade de France in Paris run onto the soccer field after explosions were heard outside the stadium on November 13, 2015. Three teams of gun-wielding ISIS militants
hit six locations around the city, killing at least 129 people and wounding hundreds.
Law enforcement officers search a residential area in San Bernardino, California, after a
mass shooting killed at least 14 people and injured 21 on December 2, 2015.
The shooters -- Syed Rizwan Farook and his wife, Tashfeen Malik -- were fatally shot in a gunbattle with police hours after the initial incident. The couple supported ISIS and had been planning the attack for some time, investigators said.
Two wounded women sit in the airport in Brussels, Belgium, after two explosions rocked the facility on March 22, 2016. A subway station in the city
was also targeted in terrorist attacks that killed at least 30 people and injured hundreds more. Investigators say the suspects belonged to the same ISIS network that was behind the Paris terror attacks in November.
A boy walks past bloodstains and debris at a cafe in Balad, Iraq, that was attacked by ISIS gunmen on May 13, 2016. Twenty people were killed.
Iraqi government forces patrol in southern Falluja, Iraq, on June 10, 2016. In late June,
a senior Iraqi general announced that the battle to reclaim Falluja from ISIS had been won.
In the ongoing offensive, forces supporting the U.N.-brokered government gained control of a port late Friday after fierce clashes with ISIS militants and are in complete control of the al-Sarawa area east of Sirte, the group added.
The offensive that has lasted almost two weeks has left more than a hundred fighters dead and about 400 others wounded, the Government of National Accord said, calling on the international community to provide urgent medical support to Libyan security forces.
The force reopened a road between Sirte and a village around 70 kilometers (about 40 miles) east after sweeping for and removing improvised explosive devices, while the country's air force carried out six raids against the militants and their weaponry in Buhari, 3 kilometers to Sirte's south, according to Al-Bunyan al-Marsous.
The advance comes as ISIS loses more ground in Iraq and Syria, with forces supported by U.S.-led air strikes slowly moving closer in on its heartland Raqqa.
Road to Raqqa: Long struggle looms in battle for ISIS heartland
Sirte, a port city on the Mediterranean coast, was best known as Moammar Gadhafi's hometown before it fell into the hands of ISIS.
The extremist group has gained a foothold in the country in a power vacuum that hasn't been filled since collapse of Gadhafi's regime collapsed in 2011.
Following the Arab Spring, there were hopes that Libya would follow a more democratic path like its neighbor, Tunisia. But warring factions soon split over how to run the country, and civil war ensued. Two rival governments claimed to be the rightful leaders before signing a U.N.-backed peace deal in December.
U.S. officials estimate there are 4,000 to 6,000 ISIS militants in the country, and there are likely to be hundreds in Sirte.
How ISIS penetrated Libya
The Pentagon is providing additional resources to counter ISIS in Libya, according to a U.S. defense official familiar with the operation.
The group first took control of much of the city of Derna, a coastal town in the east, in late in 2014, before taking Sirte in June last year.
It established a presence in Sabratha, a town not far from the Tunisian border. It has also carried out deadly terror attacks in the capital, Tripoli.
Last September, its leader there, Abu al Mugirah al Qahtani, called for more jihadists to come to Libya to spread its influence in the country, and counterterrorism officials have been concerned at the influx of fighters into Libya -- many of them Tunisians.
Libya has been an important base for ISIS in launching attacks into Tunisia and, to a lesser extent, Egypt. The operatives who carried out deadly attacks against tourists in the Tunisian resort town of Sousse last year are thought to have been trained in Libya.
Its advance in Libya has also given ISIS access to a broad swath of the Mediterranean coast, and enabled it to exploit the migrant traffic seeking to cross to Europe.
ISIS was ejected from Derna late last year and targeted in U.S. airstrikes in Sabratha last February and now seems set to lose control of Sirte. But even if it loses territory and any pretense at governing, it is likely to regroup as more of an Islamist insurgency in the vast open spaces of the Libyan desert.
As Geoff Porter, an expert in Libya and president of North Africa Risk Consulting, puts it: "The Islamic State in Libya does not have to be a replica of the Islamic State in Iraq and Syria to be dangerous and disruptive for Libya and it neighbors."