Islamic State fighters possibly seized air-defense weapons when they recaptured the ancient Syrian city of Palmyra, a US general said Wednesday, adding a potential complication to the coalition's air war.

Lieutenant General Stephen Townsend, who commands the US-led coalition bombing IS in Iraq and Syria, said jihadists seized a trove of gear when they retook the desert city from Russia-backed Syrian regime troops on Sunday.

"We believe that includes some armored vehicles and various guns and other heavy weapons, possibly some air-defense equipment," Townsend said in a video briefing from Baghdad.

The US-coalition that has been bombing IS since 2014 has enjoyed near-total air superiority across Iraq and Syria, sustaining only limited small-arms damage to a couple of aircraft.

"Basically, anything they (IS) seize poses a threat to the coalition, but we can manage those threats and we will," Townsend said.

"I anticipate that we'll have opportunities to strike that equipment and kill the ISIL that's operating it soon."

Townsend did not describe whether the air-defense weapons were anti-aircraft guns or missiles.

IS overran Palmyra on Sunday, nine months after being expelled by Russian air strikes and forces loyal to President Bashar Al-Assad.

IS had initially seized Palmyra in May 2015 and went on to blow up UNESCO-listed Roman-era temples and loot ancient relics.

Townsend said the coalition would, at least initially, defer to the Russians to try to retake Palmyra.

"If they don't, we will do what we need to do to defend ourselves and we'll de-conflict those actions with the Russians."

The Pentagon and coalition partners have accused Moscow and Damascus of taking their eyes off Palmyra to focus on bombing the former rebel bastion of Aleppo.

The US-led coalition, which is narrowly focused on fighting the IS group, has not had an active role in Aleppo, and Townsend said the regime's recapture of the city would have only a "relatively moderate" effect on coalition actions in Syria.

"Because I think the regime and the opposition forces that are fighting their war adjacent to ours will take their fight elsewhere," he said without giving details.