The Britain-based Syrian Observatory for Human Rights monitor said at least 32 fighters had been killed in some 15 strikes on the group's stronghold of Raqa province in northern Syria.
The monitor's head, Rami Abdel Rahman, said at least 40 jihadists were also wounded in the strikes, which hit IS headquarters and bases to the north, east and southeast of provincial capital Raqa city.
The city is the de facto Syrian capital of the group, which calls the large stretches of territory it controls in Syria and neighbouring Iraq an Islamic "caliphate".
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.
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The US-led coalition has been targeting IS in Syria since last September, expanding a campaign that began with raids in neighbouring Iraq.
Its operations have expanded further 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.
In an interview published Sunday in Britain's The Sunday Times newspaper, Assad slammed London's decision to begin strikes in Syria as "illegal" and said its actions would cause "terrorism" to spread.
"It will be harmful and illegal and it will support terrorism as happened after the coalition started its operation a year or so (ago)," he told the newspaper.