The two leaders agreed during a telephone call, details of which were later released by Cameron`s Downing Street office, that the use of chemical weapons would be "completely unacceptable".
Cameron and Obama agreed that "the use -- or threat -- of chemical weapons was completely unacceptable and would force them to revisit their approach so far," according to the Downing Street statement.
"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," added the statement.
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 on Wednesday, activists said, as the 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 President Bashar al-Assad`s exit in any talks on ending the brutal 17-month conflict.
In an earlier call, Cameron and Hollande "both 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," said the Downing Street statement. (AK)
Editor: Kunto Wibisono
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