It was the first time that the ISIS militants seized an entire city from Syrian government forces; it won control of its first major city, Raqqa, from Syrian insurgents and the Qaeda-affiliated Nusra Front after the two became rivals.

The rout on Wednesday in Palmyra, whose spectacular ancient ruins are a symbol of the country’s heritage embraced by supporters and opponents of President Bashar al-Assad alike, came just five days after the militants seized the much larger city of Ramadi in Iraq.

The Islamic State, also known as ISIS or ISIL, declared on Thursday that it was in control of the town after soldiers “ran away” and “left behind hundreds of dead and injured,” according to a statement released through ISIS social media channels.

Antigovernment activists who also oppose the Islamic State circulated grisly images of dead and decapitated bodies of young men lying on what looked like a street in the center of Palmyra, saying they were members of the Shueitat tribe, hundreds of whom were massacred last year for resisting the group.

The defeat is likely to increase pressure on Mr. Assad, whose forces have suffered setbacks in the northern province of Idlib in recent months and have increasingly struggled to fill their ranks after four years of war against an insurgency that began with political protest and morphed into a war with several fronts.

Although Mr. Assad still has many supporters who find him preferable to extremist groups like the Islamic State, there is increasing resentment that the state is failing to provide security.

The Islamic State militants now control about half of Syria’s territory. Nearly all of that is desert, although the militants have seized critical roads, oil and gas fields, and many sites where valuable antiquities can be excavated, now chief among them Palmyra.

Other reports from the area were hard to confirm because Internet and phone lines were disrupted. But reports from residents, activists and Islamic State supporters suggested that the group was moving to seize more of the oil and gas fields around Palmyra, a hub in a network of roads near the geographical center of the country.

There were conflicting reports about Tadmur Prison — Syria’s equivalent of Abu Ghraib in Iraq, where dissidents were long held and tortured. The spectacle of opening its doors could be a propaganda coup for the Islamic State militants, but residents said that the most high-profile political prisoners — Islamists and senior army defectors — had been moved in recent days to another prison closer to Damascus, a possible sign the government knew defeat was coming.

The Lebanese news channel MTV reported that 27 Lebanese citizens had been freed, prompting immediate speculation that they were among the Lebanese who have been missing for decades in Syria. Some inmates in Tadmur Prison have been there since a Muslim Brotherhood uprising in 1982 was crushed, and dissidents, including secular leftists, were rounded up en masse.

The report about the prison could not immediately be confirmed independently.

As Syrians woke up to the news that the Islamic State militants were in control of Palmyra, state news media focused instead on reports of victories in the Qalamoun Mountains, near the Lebanese border. Government forces there are receiving significant help on the battlefield from the Lebanese Shiite group Hezbollah.

There were no updates from the government on the situation in Palmyra. State television had broadcast patriotic music and nature scenes the night before, with state news media later reporting that “popular defense groups” had withdrawn “after securing the evacuation of most of the families.”

Residents offered a different account, however, reporting that soldiers and pro-government militiamen were fleeing the town, leaving many civilians unable to evacuate.

There have been fears locally and internationally that the Islamic State militants will harm the priceless antiquities of Palmyra. The militants have destroyed, damaged and looted other Unesco World Heritage sites in their rampage across Iraq and Syria, condemning them as idolatrous even as they loot and sell movable items to finance their activities.

While local activists said the militants were in control of the ancient site on Thursday, there was no sign they had caused damage to it. Experts said the militants might first turn their attention to the profits they could reap from Palmyra’s vast stores of unexcavated antiquities.

The group often reserves its spectacular propaganda displays, such as smashing antiquities, for when it wants to divert attention from battlefield setbacks, which was certainly not the case on Thursday.