Twenty members of the Islamic State (IS) were killed in Syrian air force raids on Wednesday morning against the militants' bastion in Raqa, a monitoring group said.
"At least 20 members of IS were killed and others were injured in air strikes... targeting an IS training base in Raqa," said the Syrian Observatory for Human Rights.
The strikes also destroyed 14 of IS' military vehicles, the Observatory added.
IS militants are firmly in control of Raqa, their stronghold, and have secured large swathes of territory in eastern Syria and in neighbouring Iraq.
While some in the Syrian opposition initially welcomed IS -- formerly known as the Islamic State of Iraq and the Levant (ISIL) -- among their ranks, viewing it as a potential ally, its abuses and quest for control turned the rebels against them.
Rebels have been fighting IS since January.
The Syrian regime of President Bashar al-Assad, for its part, has escalated its attacks against IS positions since the group spearheaded a Sunni militant offensive in neighbouring Iraq.
"From 10 June (when Mosul fell from Iraqi government control) to date, the Syrian air force really stepped up its raids against IS positions, and there have been strikes every day since then," said Observatory director Rami Abdel Rahman.
"There has been a marked escalation. The regime fears IS getting stronger, especially after it brought in vehicles from Iraq" that the militants seized from fleeing troops, he added.