31 Islamic State fighters killed in Assad air raids, reports

The Britain-based group's head, Rami Abdel Rahman, said the raids were the regime's "most intensive" against the Islamic State since IS joined the more than three-year-old conflict in Syria in spring 2003.

The IS has held most of Raqa province since February, after ousting other rebel groups battling President Bashar al-Assad's regime.

The Observatory said the regime has also carried out air strikes on the eastern province of Deir Ezzor, as well as Akhtarin and Dabeq in Aleppo province, two northern towns which the IS seized from rebels on Wednesday.

The Syrian military has now turned the bulk of its firepower on the IS since the jihadists in June declared a "caliphate" straddling Syria and neighbouring Iraq, where the group has been targeted by US air strikes over the past week. 

"The regime wants to show the Americans that it is also capable of striking the IS," said Abdel Rahman, whose group relies on information from opposition activists and medics on the ground.

"The regime strikes the IS where it is strong. In regions where the group is confronted by rebels, it doesn't intervene so that the two enemies weaken each other," he said.

"But once one party takes the upper hand, it strikes," said Abdel Rahman.

Since joining Syria's conflict, IS has also seized most of Deir Ezzor, another province on the Iraqi border, spreading terror as it imposes its own strict interpretation of Islam with penalties such as beheadings.