US-led coalition jets have bombed a Syrian army position in the country's east, killing dozens of Syrian soldiers, say Russian authorities and a monitoring group.
The Royal Australian Air Force has now admitted its forces were involved in the strike, which a Syrian army general claimed was proof the US and its allies were supporting ISIS.
Fairfax Media reports The Australian Defence Force said on Sunday that RAAF planes were "among a number of international aircraft taking part in this Coalition operation".
""While Syria remains a dynamic and complex operating environment, Australia would never intentionally target a known Syrian military unit or actively support Daesh (also known as ISIL)," a statement said.
"Defence offers its condolences to the families of any Syrian personnel killed or wounded in this incident."
It's feared the strike at Jebel Tharda near Deir al-Zor airport on Saturday will pave the way for Islamic State fighters to overrun the facility.
Syria's army general command said in a statement the attack was "conclusive evidence" of US support for Islamic State, noting that the strike was "dangerous and blatant aggression".
Islamic State said in a statement on its Amaq news channel it had gained "complete control" over Jebel Tharda.
The defence ministry in Russia, which has been aiding Syria's President Bashar al-Assad in the five-year civil war, said US jets killed more than 60 Syrian soldiers in four strikes and a fierce battle had erupted near the airport.
Syria's President Bashar al-Assad. Image: AAP
The Syrian Observatory for Human Rights, a Britain-based monitoring group with contacts across the country, cited a military source at Deir al-Zor airport as saying at least 80 Syrian soldiers had been killed in the strike.
Russia's Defence Ministry said if the coalition bombing was a mistake, it was evidence of Washington's "stubborn refusal" to co-ordinate its actions with Russia's government.
The US-led coalition has been conducting air strikes against Islamic State since September 2014 and is also supporting rebels against Assad elsewhere in Syria.
In December, Damascus accused US coalition warplanes of striking an army camp near Deir al-Zor but Washington said it was done by Russian jets.
A man carries a child on a destroyed street in Hamoryah village, in the eastern Ghouta area, a rebel stronghold east of the Syrian capital Damascus, in July, after an air strike hit the area. Image: AAP
The Observatory said Russian jets had been conducting bombing in the area at the same time, that Islamic State fighters took control of army positions on Jebel Tharda after the air strike and that clashes between them had followed.
A strike list issued by the US on Saturday said it had carried out a strike at Deir al-Zor against five Islamic State supply routes, as well as strikes near Raqqa and elsewhere in Syria.
The US military did not immediately respond to requests for comment.
Syria's army controls Deir al-Zor airport and parts of the city which are otherwise entirely surrounded by territory held by Islamic State.