President François Hollande and British Prime Minister David Cameron have spoken on the phone about the situation in Syria, in a bid to ensure that the issue remains high on the international agenda.

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During the call, Cameron and Hollande "agreed that the refugee situation was deeply troubling," with the French leader stressing that the crisis would be the focus of France's UN Security Council ministerial meeting at the end of the month," Downing Street said.

Syrian forces, backed by helicopter gunships and tanks, launched a deadly assault on parts of Damascus earlier on Wednesday, activists said, as Assad's regime battles to stamp out rebel resistance in the capital.

At least 37 people were reported killed in Damascus alone, a day after a top minister hinted that the embattled regime was ready to discuss Assad's exit in any talks on ending the brutal 17-month conflict.

Following his conversation with Hollande, Cameron spoke with US President Barack Obama, after which the British and American leaders warned that they would be forced to consider a new course of action if Syria threatens to use chemical weapons on rebel fighters.

The two leaders agreed during a telephone call that "the use – or threat – of chemical weapons was completely unacceptable and would force them to revisit their approach so far," Cameron's Downing Street office said.

"Both said that they wanted to see a credible opposition and hoped that the opposition would use their upcoming meeting in Cairo to show real unity of purpose and coherence in working towards transition."

The White House said Obama conveyed to Cameron his concern about the "increasingly dire" humanitarian situation in Syria, and called for more countries to contribute to humanitarian appeals from the United Nations.

"The two leaders exchanged views on ways the international community can assist those displaced by the conflict," and "apply pressure" on President Bashar al-Assad's regime, it added.

On other matters, Obama expressed his "strong support for decisive action" on the economic crisis rattling Europe.