BAGHDAD -- Islamic State militants tightened their hold on the Syrian city of Palmyra yesterday, leaving decapitated bodies of suspected opponents in the streets as announcements from mosque loudspeakers urged residents to turn in government soldiers.

The militants' capture of Palmyra has raised concerns about the fate of the ancient ruins in the city, a UNESCO World Heritage site about 130 miles northeast of the Syrian capital, Damascus. Activists also have warned that a large civilian population remains in Palmyra, despite government claims that its fighters secured the residents' exit before retreating.

The militants' advance came less than a week after the Islamic State seized the Iraqi city of Ramadi, undermining U.S. assertions that the group is largely on the defensive after months of airstrikes by an international coalition. The group continued to gain ground east of Ramadi yesterday.

Palmyra's fall also marks the first time that Islamic State forces have seized a major population center directly from the Syrian government. Previous advances came against rebel groups.

In a statement released on affiliated social media sites, the Islamic State said it had taken complete control of the city, including its notorious prison and military airport. Pro-government forces "collapsed and fled," it said.

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A day after Palmyra fell, mosques announced that families hiding regime soldiers should turn them over to Islamic State authorities, activists said.

As militants searched door to door, grisly images showing beheaded bodies on the city's streets circulated on social media. The dead were said to be members of the Shaitat tribe. The tribe paid a heavy price for rising up against the Islamic State last year; as many as 700 members were massacred in a failed revolt.

The Islamic State's latest gains give the group control of a key route to Damascus, while cutting supply lines to beleaguered Syrian forces farther east in Deir al-Zour province. The advances also consolidate the group's control along the Iraqi border.