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The Syrian regime claims to have launched a counter attack against the Islamic State group in Deir az-Zour following a mistaken US-led coalition bombing of an army position.

Damascus said it launched the attack on IS militants close to the key airbase, where the extremists have surrounded regime positions.

On Saturday, the Pentagon said that coalition pilots had believed mistakenly bombed the regime army, killing dozens of troops.

Pilots halted the raid as soon as Damascus ally Moscow informed commanders that army positions were coming under attack.

Damascus has reacted angrily to the deadly strike, which forced troops to pull back from two strategic hilltops overlooking the besieged airbase on the outskirts of the city of Deir az-Zour.

"The Syrian army has returned to the offensive," a regime official told AFP.

"After the American raids, it withdrew from several positions but now it has gone back on the attack."

Lost ground

Regime troops at Deir az-Zour airbase said that troops have regained some of the lost ground.

"The army has retaken most of its positions on Jabal Therdeh with Russian and Syrian air support," the source said, referring to one of the two hills lost on Saturday.

"The two countries' air forces bombed the area around the airbase, neighbourhoods held by the jihadists and the road linking Deir az-Zour to Mayadeen," an IS-held town 45 kilometres (30 miles) to the southeast, the source added.

The mountain overlooking the airbase is vital for the army as control of them would allow IS to fire on all aircraft trying to take off or land.

The airbase and adjacent government-held neighbourhoods of the Deir az-Zour city have been under siege since 2012 and have been dependent on resupply by air.

The Syrian Observatory for Human Rights said at least 30 jihadists were killed in Sunday's counterattack by the army.

The UK-based monitoring group said 90 soldiers were killed in Saturday's air strike, sharply higher than the death toll of 62 given by Moscow on Saturday.

'Unfortunate incident'

The US ambassador to the United Nations, Samantha Power, voiced regret for the loss of life.

"If we determine that we did indeed strike Syrian military personnel, that was not our intention. And we of course regret the loss of life," she said.

Australia, which said it was one of several coalition countries whose aircraft took part, offered its "condolences to the families of any Syrian personnel killed or wounded".

"While Syria remains a dynamic and complex operating environment, Australia would never intentionally target a known Syrian military unit or actively support Daesh (IS)," a statement from the military said on Sunday.