DAMASCUS, Syria -- Fears mounted over the fate of one of the Mideast's most prominent archaeological sites after Islamic State militants overran the historic Syrian town of Palmyra, seizing control Thursday of its temples, tombs and colonnades within hours.
The takeover also expanded the extremists' hold, making them the single group controlling the most territory in Syria.
"The Syrian regime appears to be in terminal decline, and the Islamic State group in its timing is capitalizing on recent losses by government forces in the north and south," said Amr Al-Azm, an antiquities expert and professor at Shawnee State University in Ohio.
The militants overran the famed archaeological site early Thursday, just hours after seizing the nearby town in central Syria, activists and officials said.
They also captured Palmyra's airport and the notorious Tadmur prison, delivering a startling new defeat for President Bashar Assad, whose forces quickly retreated. Hundreds of Palmyra residents fled the town of 65,000, and many more were trying to escape, said Talal Barazi, the governor of central Homs province, which includes Palmyra.
An oasis set in the Syrian desert, Palmyra is a strategic crossroads linking the capital Damascus and cities to the east and the west. Its capture raised alarm over some of the world's most important ancient ruins, whose fate remained unknown Thursday, and no photos or video emerged from the militants.
"We are in a state of anticipation and fear," said Maamoun Abdulkarim, the head of the Antiquities and Museum Department in Damascus. "The city is now totally controlled by gunmen and its destiny is dark and dim."
A UNESCO world heritage site, Palymra boasts 2,000-year-old towering Roman-era colonnades, temples and priceless artifacts that have earned it the affectionate name among Syrians of the "Bride of the Desert."
They are the remnants of an Arab client state of the Roman Empire that briefly rebelled and carved out its own kingdom in the 3rd Century, led by Queen Zenobia, with Palmyra as its capital. Before the war, it was Syria's top tourist attraction, drawing tens of thousands of visitors each year.
It includes a 3,000-seat amphitheatre overlooking a colonnaded main avenue where plays, concerts and youth festivals were staged.
With the capture of Palmyra, the Islamic State militants now control half of Syria and most of the country's oil wells, according to the Syrian Observatory for Human Rights, making it the group with the most territory under its authority among the myriad factions fighting in the country's civil war.
Its vast terrain inside Syria stretches from the group's westernmost strongholds in Aleppo province to its core territory in northeastern Syria down to central Syria, with footholds in Damascus.
Palmyra's location in Syria's heartland offers the militants several important advantages, said Faysal Itani, a resident fellow at the Atlantic Council. The town can now be used as a launching pad to threaten government positions and supply lines south of Aleppo and east of Homs and Hama, and open up a new approach to Damascus, seat of Assad's power.
IS can also threaten regime supply lines to the eastern city of Deir el-Zour, where government forces are still holding out against the militants.
"If IS manages to cut off Deir el-Zour, it is likely that the city would fall, essentially ending regime presence in that province, and consolidating IS' core territory," he said.
The fall of Palmyra follows major setbacks for Assad in northern and southern Syria. "This is simply an indication of how overstretched the regime is," Itani said.
The militants' capture of Palmyra came just days after Islamic State fighters seized the strategic Iraqi city of Ramadi, illustrating the extremists' ability to advance on multiple fronts at opposite ends of a sprawling battlefield that spans the two countries, where it has declared a caliphate or Islamic state on the territory it controls.
At the White House, Press Secretary Josh Earnest described the developments in Palmyra and Ramadi as setbacks, but insisted the U.S.-led air campaign was making progress overall "in degrading ISIL capabilities."
The head of the U.N.'s cultural agency called on Syria's warring factions to immediately end hostilities around the archaeological site.
"I am extremely worried about what happens in Palmyra," UNESCO chief Irina Bokova said. "Palmyra is an extraordinary world heritage site in the desert and any destruction to Palmyra is not just a war crime, but ... an enormous loss to humanity."
Al-Azm said he expected the militants to turn to looting and excavating the town's antiquities, selling the artifacts on the black market before eventually destroying the site.
In taking Palmyra, IS also seized control of the notorious Tadmur Prison, freeing some of those imprisoned inside, said Bebars al-Talawy, a Homs activist. The government had already transferred thousands of detainees from the prison to a jail near Damascus as IS attacked the city, al-Talawy said. Thousands were believed to still be inside, he said, but he couldn't provide precise figures.
The prison survives in the collective memory of Syrians as the place where dissidents were held for decades and prisoners tortured.
A report by a local Lebanese station that 27 Lebanese prisoners, including some jailed since the 1980s, were among those freed from Tadmur triggered confusion in Beirut among families of missing Lebanese, many of whom believe their loved ones have been languishing in Syrian prisons for decades.
An amateur video posted online purported to show IS fighters inside the Tadmur Prison setting fire to a giant poster of Assad and cheering. The video and its location could not be independently verified but appeared genuine and corresponded to other AP reporting of the events.
Palmyra's fall came at a deadly toll.
The Observatory said Thursday that according to its estimates, 462 people have been killed since IS began its offensive on Palmyra and nearby areas on May 13. It said the dead included 241 troops and pro-government gunmen, as well as 150 IS fighters. The rest were civilians, presumably killed by IS or in cross-fire.
Meanwhile, Islamic State militants made further inroads in Iraq's Anbar province, seizing the Iraqi side of a key border crossing with Syria after Iraqi government forces pulled out, according to Athal al-Fahdawi, a local councilman. The fall of the al-Walid crossing will help the militants to shuttle weaponry and reinforcements more easily across the Iraqi-Syrian borders.
Despite the Islamic State's stunning victories in Palmyra and Iraq, the extremists suffered a setback in Syria's northeastern province of Hassakeh, where they have come under attack by Kurdish fighters.
The Kurdish fighters captured much of the Abdul-Aziz Mountain near the village of Tel Tamr on Wednesday, according to the Observatory and the Kurdish forces known as the People's Protection Units, or YPG.
The Observatory said YPG fighters were backed by airstrikes of the U.S.-led coalition, which has been bombing IS positions in Syria since September.
Karam reported from Beirut. Associated Press writers Bassem Mroue in Beirut and Sameer Yacoub in Baghdad contributed to this report.
