At least 32 Islamic State (IS) fighters were reportedly killed by the US-led coalition air strikes on IS' Syrian stronghold of Raqa province on Sunday.
AFP reported Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, saying that more than 40 IS jihadists were also wounded in a series of around 15 strikes on IS bases by planes believed to be from the US-led coalition".
He said the wave of raids hit IS bases to the north, east and southeast of Raqa city, which is the group's de facto Syrian headquarters.
Rahman said the casualty figures were collected from a single hospital, while the final toll from the air strikes could rise.
The US-led coalition has been carrying out strikes against IS in Syria since last September, expanding a campaign that began with raids in neighbouring Iraq.
IS controls large stretches of territory in the two countries, which it describes as an Islamic "caliphate".
The US-led coalition has expanded its operations in recent days, partly in response to the deadly attacks in Paris claimed by IS.
Britain voted on Wednesday to join the coalition's strikes in Syria, after a heated debate in the country's parliament and with the staunch backing of Prime Minister David Cameron.
And German lawmakers on Friday approved plans to join the military action against the group in Syria.
Raqa is frequently the target of air strikes by the US-led coalition, as well as the Syrian air force and Russian warplanes that began an air campaign in Syria in late September.
