DAMASCUS, Syria — Islamic State militants overran the famed archaeological site at Palmyra early on Thursday, just hours after seizing the central Syrian town, activists and officials said, raising concerns the extremists might destroy some of the priceless ruins as they have done in neighboring Iraq.

The Islamic State’s capture of the town of Palmyra late Wednesday was a stunning triumph for the militant group, only days after it captured the strategic city of Ramadi in Iraq’s largest Sunni province.

As IS took Palmyra, government forces collapsed in the face of the attacks and Syrian soldiers were seen fleeing the area, activists said.