At least 32 Islamic State of Iraq and the Levant (ISIL) fighters have been killed in apparent US-led coalition raids, as Syria's President Bashar al-Assad slammed Britain's decision to join the fight.
The Britain-based Syrian Observatory for Human Rights on Sunday said the ISIL members were killed in about 15 strikes on the group's stronghold of Raqqa province in northern Syria.
It will be harmful and illegal and it will support terrorism as happened after the coalition started its operation a year or so [ago].
The monitoring group's Rami Abdel Rahman said the strikes hit ISIL headquarters and bases to the north, east and southeast of Raqqa city, the group's de facto capital.
Raqqa is frequently the target of air strikes by the US-led coalition, as well as the Syrian air force. Russian jets began an air campaign in Syria in late September and also target the city.
The US-led coalition has expanded its operations in recent days, partly in response to the Paris attacks.
Britain voted on Wednesday to join the coalition strikes in Syria, after a heated debate in its parliament and with the strong backing of Prime Minister David Cameron.
In an interview published on Sunday in Britain's Sunday Times newspaper, Assad called the UK strikes "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 said.
"Terror", he said, was like a cancer which needed to be tackled with a comprehensive strategy and that would involve working with troops on the ground.
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"You cannot cut out part of the cancer. You have to extract it. This kind of operation is like cutting out part of the cancer. That will make it spread in the body faster."
Britain began its bombing on Thursday, hitting an ISIL oilfield just hours after the parliamentary vote.
Damascus has repeatedly said the US-led coalition is ineffective and illegal, insisting it cannot uproot ISIL without coordinating with the Syrian government.
In making the case for military action, Cameron said there were 70,000 moderate Syrian fighters on the ground who could help secure territory cleared by air strikes.
Assad ridiculed the figure, called it "classical farce".
"Where are the 70,000 moderates he is talking about? There is no 70,000. There is no 7,000," he said.