A UN plan for Syria is a "milestone" in the efforts to end the conflict there, says US Secretary of State John Kerry.
He said the plan gave Syrians a "real choice... between war and peace".
However, disagreements remain over President Assad and the unanimously agreed Security Council resolution makes no mention of his future role.
Western countries have called for his departure, but Russia and China say he should not be required to leave power as a precondition for peace talks.
Mr Kerry said Mr Assad had "lost the ability to unite the country".
But he also said that demanding Mr Assad's immediate departure was "prolonging the war".
"We are under no illusions about the obstacles that exist," Mr Kerry said. "There obviously remain sharp differences within the international community, especially about the future of President Assad."
However, while Western and Arab nations accept that Mr Assad can be part of the transition, they insist he must be gone at the end of it.
France's Foreign Minister Laurent Fabius said the idea of Mr Assad standing in elections was "unacceptable".
British Foreign Secretary Philip Hammond said it would be "impossible to bring peace and unity while he remains".
But Syria's UN ambassador Bashar Jaafari said there was a "glaring contradiction" between outside countries talking about letting Syrians decide their future yet also insisting on replacing Mr Assad.