The UN Security Council has unanimously agreed on the text of a draft resolution for peace talks in January and a ceasefire aimed at ending the war in Syria.
All 15 members of the council agreed to approve the text, which came after hours of negotiation between world powers on Friday.
Al Jazeera's Diplomatic Editor James Bays, reporting from New York, said the text was a long time coming and came despite reservations from Russia.
"There was lots of resistance from Moscow but Western diplomats [are] telling me they are very happy with what they have," he said.
"This will be the first time there's been a Security Council resolution on Syria and on political transition in Syria during five years of civil war."
John Kerry, US secretary of state, said the resolution aims to install a transitional government within the first six months, paving the way for elections within the next 18 months.
He said any agreement would exclude the Islamic State of Iraq and the Levant (ISIL) and al-Nusra Front.
"We have emphasised from the beginning that for this to work it has to be implemented by the men and women of Syria and cannot be opposed from the outside," Kerry said.
"The resolution that we just approved is a milestone because it sets out specific concepts with specific time-frames.
The text does not include mention of Assad's fate or which opposition groups will be included in peace talks.
Both have been key obstacles in reaching an agreement in talks thus far.
Earlier on Friday, US President Barack Obama reaffirmed his insistence that Assad step down, warning there can be no peace there without a legitimate government.
"I think that Assad is going to have to leave in order for the country to stop the blood, for all the parties involved to be able to move forward in a nonsectarian way," Obama said at a year-end news conference at the White House.
The conflict in Syria started in mid-2011 after protests against Assad's rule were violently put down by government security forces.
The war has killed more than 300,000 people according to some estimates, and sent millions of Syrians fleeing for neighbouring countries and Europe, giving rise to the worst refugee crisis since World War II.
Groups such as ISIL have also conquered large tracts of land, from where they have planned attacks on a number of states including France, Turkey, and Lebanon.
Source: Al Jazeera