Islamic State has released new footage depicting unscathed ruins in the Syrian city of Palmyra, and activists say the group has promised to spare much of the ancient site and destroy only statues deemed polytheistic – raising hopes that some of the most magnificent surviving ruins of antiquity may remain intact.

But the human toll of the Syrian conflict increased as president Bashar al-Assad’s air force responded to the loss of Palmyra with an unforgiving air campaign that killed more than a dozen civilians on Monday and appears to have triggered a fresh humanitarian crisis in the area.

The air strikes came after the regime abandoned Palmyra and most of its civilians with Islamic State (IS) at the city gates, after initially saying it had evacuated non-combatants.

Activists and a monitoring group said IS had used the majestic Roman theatre in Palmyra to execute nearly two dozen pro-Assad foreign fighters on Tuesday. The group held nearly 600 prisoners that it said had fought alongside the regime.

More than 200 people have been killed since the IS offensive in the area began in mid-May.

An 87-second video released by one of the militant group’s media arms on Wednesday shows parts of the ancient city’s ruins and colonnades – the Temple of Bel and the Roman theatre – apparently undamaged and empty.

“They haven’t been damaged and members of the organisation told residents that they will not damage the city’s antiquities, but will destroy the idols,” an activist with the anti-regime Local Co-ordination Committee for Tadmur, the modern name for Palmyra, said. “Perhaps it’s because the Palmyra antiquities are mostly columns and large buildings and not statues of people, which they consider idols that must be destroyed, and they have no problem with the other antiquities.”

Alwan FM, an anti-regime Syrian radio station, published an audio interview purporting to be with Abu Laith al-Saoudy, the nom de guerre of the IS military commander in Palmyra, who pledged not to damage the site but said the group would destroy statues.

“Concerning the historic city, we will preserve it and it will not be harmed, God willing,” he said. “What we will do is break the idols that the infidels used to worship. The historic buildings will not be touched and we will not bring bulldozers to destroy them like some people think,” he added.

There were fears for the magnificent ruins of the city, an ancient silk-road hub and one of the best-preserved sites from antiquity, after Islamic State routed the regime garrison last week following a seven-day siege.

The regime air raids have forced many civilians to try to escape the city’s bombardment and increased the number of internal refugees in the area.

The Assad regime had claimed that it evacuated civilians from the city as its forces withdrew, before contradicting its own claims and announcing over the weekend that Islami State had killed 400.

“I don’t think there’s any point in discussing the regime’s credibility,” the activist said.

Guardian service