The air strike killed Syrian soldiers and was "conclusive evidence" that the U.S. and its allies support the jihadist group, the Syrian army said in a statement, noting that the strike was "dangerous and blatant aggression".

The U.S.-led coalition has been conducting air strikes against Islamic State since September 2014. In December Damascus accused the coalition of striking an army camp near Deir al-Zor, but Washington said it was done by Russian jets.

A strike list issued by the U.S. 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.

Syria's army controls Deir al-Zor airport and parts of the city which are otherwise entirely surrounded by territory held by Islamic State.

The United States and Russia agreed a deal on Syria last week, involving a ceasefire which came into effect on Monday, aid deliveries to besieged areas and eventual joint targeting of militant jihadist groups if the truce works out.