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”.
The resolution, passed unanimously by the UN Security Council, sets out a timetable for formal talks and a unity government within six months,
However, the resolution makes no mention of the future role of Syrian President Bashar al-Assad.
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”.
However, while Western and Arab nations accept that Mr Assad can be part of the transition, they also 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”.
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.
The resolution foresees talks between the Syrian government and opposition in early January.
Russian Foreign Minister Sergei Lavrov said it would be possible to reach an agreement on a unity government for Syria within six months.
The plan sets out a timetable for UN-supervised elections within 18 months, and stresses that the Syrian people will decide the future of their country.
The plan also calls for a ceasefire, but there is also disagreement over which armed groups should be designated as terrorists and consequently excluded from any talks or ceasefire.
Actions against groups considered terrorist organisations would not be affected, allowing Russian, French and US air strikes against Islamic State to continue.
The agreement demands that all parties cease attacks against civilians.
The Syrian war, which is heading towards its fifth year, has killed more than 250,000 people and displaced millions more, the UN says.