At least 31 Islamic State militants and eight civilians died in Syrian air force raids Sunday in the northern province of Raqa, a stronghold of the jihadist fighters, a monitoring group said.
The air force carried out 16 raids on the city of Raqa and several more on the town of Tabqa in Raqa province, killing at least 31 jihadists and eight civilians, said the Syrian Observatory for Human Rights.
The Britain-based group's head, Rami Abdel Rahman, said the raids were the regime's "most intensive" against the Islamic State since the jihadists joined the more than three-year-old conflict in Syria in spring 2013.
The IS has held most of Raqa province since February, after having ousted other rebel groups battling President Bashar al-Assad's regime.
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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 turned 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.