Islamic State militants tightened their hold on the Syrian city of Palmyra on Thursday, dumping decapitated bodies of suspected opponents in the streets and blaring calls from mosque loudspeakers for residents to turn in government soldiers.

The militants’ capture of Palmyra, about 130 miles northeast of the Syrian capital, has raised global alarm about the fate of the ancient ruins in the city, a UNESCO World Heritage site. It also subjected yet another urban population to the harsh rule of the Islamist extremists, activists said, explaining that tens of thousands of people remained in the city — despite government claims that its fighters had enabled citizens to evacuate before the soldiers retreated.

The militants’ advance came just days after the Islamic State seized the key Iraqi city of Ramadi, undermining U.S. assertions that the group is largely on the defensive after months of airstrikes by an international coalition. While the Iraqi government has vowed to take back Ramadi, Islamic State militants continued to gain ground east of the city on Thursday.

The jihadists’ new gains mark a significant propaganda victory for Islamic State, and a blow to a U.S.-backed military campaign that had seemed to be making progress with Iraqi forces’ success last month in pushing the extremist fighters out of the city of Tikrit.

President Obama said in an interview published Thursday that the United States has to “ramp up not just training, but also commitment” to the pro-government forces fighting in Iraq. The Pentagon announced it would rush 2,000 anti-tank weapons to Iraqi forces.

The Islamic State says it has taken full control Palmyra, a UNESCO World Heritage site in Syria, just days after capturing Ramadi, a provincial capital in Iraq. (Reuters)

In Syria, the Islamic State’s latest gains give the al-Qaeda offshoot, which is also known as ISIS or ISIL, control of a key route to the capital, 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 border with Iraq, where its fighters on Thursday seized the only crossing point they did not control after government forces pulled out of al-Tanf, according to the Britain-based Syrian Observatory for Human Rights.

Palmyra’s fall 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 Washington, White House spokesman Josh Earnest acknowledged Thursday that the capture of Palmyra was a “setback” for the U.S.-led coalition fighting the extremists. But, even as the administration came under harsh criticism from Republicans, the spokesman said Obama was not planning any major military deployment to Iraq or Syria.

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

A day after the city fell, orders rang from mosque loudspeakers Thursday morning telling families hiding regime soldiers to turn them over to Islamic State authorities, activists said.

Palmyra seized by the Islamic State

As militants searched house by house for their opponents, grisly images circulated on social media showing beheaded bodies on the city’s streets. 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.

Activists estimate that more than 100,000 civilians may still be in the city, whose population had swelled because of an influx of internally displaced people from other parts of Syria.

There were no immediate reports of damage to Palmyra’s famed archaeological site, including the remains of temples and artifacts that testify to the area’s rich legacy as a commercial and political crossroads dating back more than 2,000 years.

The Islamic State, ostensibly acting in the name of religious purity, has destroyed pre-Islamic treasures in northern Iraq that it deemed blasphemous, including other UNESCO heritage sites such as Hatra and Nimrud.

For the moment, the group is more focused on rooting out government fighters and collaborators in Palmyra, as well as securing the city militarily, activists said.

It was more than a month after the capture of Mosul in June 2014 that Islamic State militants destroyed the city’s ancient tomb of Jonah, and the media-conscious group is likely to carefully weigh when it can best benefit from publicity before destroying Palmyra’s historic sites.

Experts have warned that smaller items that can be looted and smuggled are likely to be sold off by the group.

“At risk are the magnificent structures visible above ground and also the untold numbers of invaluable artifacts that lie unexcavated beneath the surface, ripe for plucking by plunderers who would sell them to fund ISIS operations,” said Carol Meyers, a professor emeritus of religion at Duke University.

Palmyra’s archaeological site had not been damaged as of Thursday, according to activists. Islamic State militants “just don’t have time for it now,” said a Syrian activist in close contact with sources in the city, speaking on condition of anonymity for security reasons. “They are more into the prison and the stocks of the missiles and other military facilities.”

In addition to the rich cultural heritage, Palmyra’s environs are rich in gas and oil, potentially boosting the Islamic State’s wealth.

The Islamic State-linked Amaq news agency claimed Thursday that about 75 percent of Syria’s electric and gas resources are now in the group’s hands. The gains in the vicinity of Palmyra included the seizure of an oil pumping station that supplies the country’s main port in Tartus. Amaq claimed that more than 40 Syrian soldiers were killed at the pumping station.

Palmyra’s loss is expected to increase pressure on President Bashar al-Assad, whose forces have been losing ground to other armed groups fighting to overthrow him. Syrian state television carried little coverage of the fall of Palmyra. The official Syrian Arab News Agency accused Western nations of “standing still” and doing nothing to protect the city.

Sam Rifaie in Beirut contributed to this report.

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