Islamic State militants seized control of the majority of the Syrian city of Palmyra on Wednesday, marking the second significant strategic gain for the group in the past week and leaving one of the region’s most renowned archaeological sites in peril.

Activists and Syrian state media said pro-government forces had withdrawn from the city 130 miles northeast of Damascus after a week-long assault by the militants. The city’s notorious Tadmor Prison, where scores of anti-regime political prisoners are incarcerated, was also in the extremist group’s hands by nightfall, activists said.

The gain consolidates the Islamic State’s control west toward the Syrian capital and east in the direction of the border with Iraq, where militants seized the city of Ramadi on Sunday. Advances by the Islamic State demonstrate the group’s ability to continue to take territory, despite recent assertions by American officials that it remains largely on the defensive after 10 months of U.S.-led airstrikes.

The fall of Palmyra to Islamic State forces effectively puts its ancient sites, which lie just on the outskirts of the modern city, in the group’s hands. Irina Bokova, ­director-general of UNESCO, said she was “ deeply concerned” about the situation at the site, which rose to prominence as a wealthy caravan oasis in the 1st century A.D. After some 2,000 years, the striking Roman colonnades of the Temple of Baal still stand majestically in the desert.

A video purports to show smoke rising over the Syrian city of Palmyra amid reports that Islamic State fighters have seized around a third of the historic city that is home to ancient ruins that date back some 2000 years. (Reuters)

Since its advances in Iraq last summer, the Islamic State has laid waste to sites dating to antiquity, branding them heretical according to its interpretation of Islam. Its fighters have smashed statues and buildings and sold off ancient artifacts that were small enough to be smuggled. The area surrounding Palmyra is also rich in gas and oil, potentially boosting the Islamic State’s wealth.

The “vast majority” of the city is in Islamic State hands, said a Syrian activist who is in contact with sources in the area. He spoke on the condition of anonymity for security reasons.

Clashes were continuing at the city’s military intelligence headquarters, while Islamic State fighters had surrounded the military airport and seized several weapons depots, he said.

An activist in Palmyra who for security reasons uses the pseudonym Ahmed al-Homsi said the Islamic State had released prisoners from Tadmor. The Syrian government’s warplanes were carrying out airstrikes Wednesday night, he said.

The capture of Palmyra, home to roughly 50,000 people, is one of the Islamic State’s first significant advances directly against President Bashar al-Assad’s forces. Islamic State fighters had previously won ground in Syria mainly from rebel groups.

Pro-government defense units had withdrawn from Palmyra after evacuating civilians following an attack during which Islamic State militants attempted to enter the city’s archaeological sites, the Syrian state news agency SANA reported.

Hundreds of ancient statues were removed from Palmyra for safekeeping as the militants closed in, Syrian officials told the news agency. Pro-government forces had largely pulled back 25 miles southwest to Sawwana, activists said.

A general view taken on May 18, 2015, shows the ancient Syrian city of Palmyra, shortly before it was seized by Islamic State fighters. (Str/AFP/Getty Images)

Fall of Ramadi reflects failure of Iraq’s strategy against ISIS

The hidden hand behind the Islamic State militants? Saddam Hussein’s

Assad’s hold on power looks shakier than ever