Cameron, who has just returned to Westminster after the summer break, spoke separately to French President Francois Hollande in a bid to ensure that Syria remained the focus of international attention.
Syrian forces backed by helicopter gunships and tanks launched a deadly assault on parts of Damascus earlier, 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.
In an earlier 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.
Source: agencies
