BEIRUT, Lebanon — Islamic State militants swept into the historic desert city of Palmyra in central Syria on Wednesday, and by evening were in control of it, residents and the Syrian state news media said, a victory that gives them another strategically important prize five days after the group seized the Iraqi city of Ramadi.
Palmyra has extra resonance as home to some of the world’s most magnificent remnants of antiquity, as well as the grimmer modern landmark of Tadmur Prison, where Syrian dissidents have languished over the decades.
But for the fighters on the ground, the city of 50,000 people is significant because it sits among gas fields and astride a network of roads across the country’s central desert.
As they have swept across Syria and Iraq, Islamic State fighters have destroyed or damaged numerous ancient sites and sculptures, condemning them as idolatry in slickly produced recruitment films, even as they pillage and sell off more portable items to finance their activities. That has raised fears both locally and internationally that Palmyra, a United Nations world heritage site, could also be irrevocably damaged.
Aside from the threat of destruction, Palmyra’s vast unexcavated antiquities could also provide significant revenue through illegal trafficking.
“The fighting is putting at risk one of the most significant sites in the Middle East,” Irina Bokova, director general of Unesco, said in a statement Wednesday.
As the city’s defenses crumbled, residents described panicked scenes of soldiers and the police fleeing, wounded civilians unable to reach hospitals and museum workers hurrying to pack up antiquities.
The loss of Palmyra, just as the United States is scrambling to come up with a response to the loss of Ramadi, the capital of Iraq’s Anbar Province, is sure to renew doubt about the Obama administration’s plans to defeat the Islamic State, also known as ISIS or ISIL.
The two successes, at opposite ends of a battlefield sprawling across two countries, showed the extremist group’s ability to shake off setbacks and advance on multiple fronts, less than two months after it was driven from the Iraqi city of Tikrit — erasing any notion that the group had suffered a game-changing blow there.
In Iraq, the fall of Ramadi has left the United States military in the uncomfortable position of supporting an attempt to reclaim the city, in Iraq’s Sunni heartland, with the help of Iranian-backed Shiite militias whose participation there Washington had previously opposed.
In Syria, a new awkwardness arises. Any airstrikes against Islamic State militants in and around Palmyra would probably benefit the forces of President Bashar al-Assad. So far, United States-led airstrikes in Syria have largely focused on areas far outside government control, to avoid the perception of aiding a leader whose ouster President Obama has called for.
There have also been calls from international cultural organizations to protect the ruins — although how that could be done was unclear — to prevent a repeat of Islamic State attacks on other renowned ancient sites in recent months.
The fall of Palmyra has also brought to a head, in a new way, the dilemma of Syrians who oppose both the Islamic State and Mr. Assad. The city was partly held for a time by local rebel fighters, before the Islamic State took shape as a major player in the conflict. But they no longer have a presence there, putting some of Mr. Assad’s opponents in the odd position of hoping that his forces can protect the city, and the ruins.
“It’s the elephant in the room,” said Amr al-Azm , a former Syrian antiquities official who teaches at Shawnee State University in Ohio. He has joined calls in recent days for international protection for the ruins. That, he said, would effectively mean a military intervention aiding the government he fervently opposes.
“I’m really frustrated that I’ve been reduced to this situation,” he said, complaining about Mr. Assad’s success in “pushing this binary on us Syrians and the international community: There is no alternative; it’s either us or a far worse threat.”
But no intervention appears likely. People in Palmyra, a relatively remote city, its population swollen with tens of thousands of displaced Syrians, were left on their own, squeezed between government forces and the Islamic State.
Residents said that by nightfall, the Islamic State had seized most of the city and was even distributing bread to some residents. Soldiers and the police could be seen fleeing, they said, prompting one cafe owner to exclaim over the phone: “Treason! It’s treason.”
Soon after government forces left the city, airstrikes began, residents said.
Workers could be seen earlier Wednesday packing up four truckloads of small boxes from the museum on the edge of the ruins, apparently carting away more antiquities in addition to items already removed for safekeeping, said Khaled al-Homsi, a Palmyra resident and anti-government activist who documents damage to the site by combatants.
The Islamic State was coming closer, he said, as a squad of 12 soldiers who had manned a nearby checkpoint appeared to withdraw. As he spoke over Internet chat, a boom could be heard; he said government airstrikes were coming dangerously close to the archaeological site’s medieval citadel.
“It’s bad today,” Khalil al-Hariri, the museum’s director, said in a brief telephone conversation, while Syria’s top antiquities director told Reuters that hundreds of objects were being moved to safety. Another museum employee, who had earlier vowed not to leave, said by phone, “Pray for us.”
The ancient site is cherished by Syrians on both sides of the original conflict between Mr. Assad and his opponents, which began with political protests in 2011 and metastasized into a multifront war.
Local rebels — early in the conflict, before the Islamic State appeared on the scene — once called themselves Grandchildren of Zenobia, referring to an ancient queen of Palmyra who briefly ruled an empire stretching from Egypt to modern-day Ankara, the Turkish capital. In the recent fighting, some government troops had vowed in social media posts that “Zenobia will never fall.”
In battles overnight, the militants captured several important locations in the northern part of Palmyra, including two security facilities and the public central bakery, according to anti-government activists, who said they had not yet entered the ruins.
“I’m here and still breathing,” Mr. Homsi, who uses a nom de guerre for his safety, said in a text message earlier Wednesday.
At the same time, he lamented that more attention had been focused on the threat to the city’s ruins than to its residents. Islamic State fighters massacred captured soldiers and civilians in outlying villages last week, according to Mr. Homsi and a government soldier whose comrades were killed. And Mr. Homsi said several civilians had been killed by government shelling in Palmyra, unable to get treatment at the hospital, which was being used solely for military casualties.
Several Palmyra residents said on Wednesday that they were staying indoors and hoping to stay out of the fighting or politics. Asked what he would do if forced to choose between the government and the Islamic State, Mr. Homsi was silent for several seconds. Finally he said, “I will try to remain neutral.” On Wednesday night, he was keeping his head down.
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