"In January we hope and expect to be at the table and to be able to implement a full ceasefire," US Secretary of State John Kerry told reporters.
"And that means all the barrel bombs will stop, all the bombing, all the shooting, all the attacks on either side."
The Security Council met Friday after the latest round of talks by the International Syria Support Group (ISSG), which had gathered in New York to renew its push for peace.
Attention now turns to Moscow and Riyadh, as Russia pressures Assad's regime to agree to a ceasefire and Saudi Arabia wrangles the opposition to form a negotiating team.
The UN special envoy on the conflict, Staffan de Mistura, said he would send out invitations to talks in January.
"Let's be realistic," he said, explaining a decision to push back the planned date of talks from January 1 to later in the same month.
"We intend and hope... that we will be able to do it in January. We will be aiming at that."
The United States and Arab allies remain convinced Assad must leave office as part of the process, but his ally Moscow insists this is a decision for the Syrian people.
The resolution does not touch on this vital issue.
- 'Massacred so many' -
"Only a Syrian-led, inclusive dialogue can put an end to the untold suffering of the Syrian people," Russian Foreign Minister Sergei Lavrov warned the Security council.
Nevertheless, Kerry -- who has "agreed to disagree" with Moscow on Assad's fate -- said the vote would act as a springboard for forging ahead with peace talks.
He emphasized that victory over the Islamic State group, also known by the Arabic acronym Daesh, hinges on a peaceful settlement in the broader Syrian civil war.
"We know that Daesh can never be allowed to gain control in Syria so we have a global imperative here to deal with a terrorist entity but also to end the civil war," he said.
"President Assad in our judgment... has lost the ability, the credibility to be able to unite the country and to provide the moral credibility to be able to govern it."
Kerry said that most ISSG members agreed that Assad would have to go, and many said so, notably France.
"How could somebody bring together a whole people when he has massacred so many?" Foreign Minister Laurent Fabius demanded.
The resolution enshrines the plan developed by the ISSG countries in a series of meetings in Geneva and Vienna.
"We have a timeframe. That transitional process needs to try to be achieved within the target time of six months," Kerry said, arguing that the UN vote gave the plan legal force.
"The election needs to take place within 18 months, the same start time, so that after six months of the transition you're about a year away, or less, from an election."
- Assad's fate -
More than 250,000 people have died since Syria's conflict erupted in March 2011, and millions more have fled their homes.
Friday's New York talks were the first by the ISSG since Saudi Arabia gathered a coalition of Syria rebel groups to form an opposition negotiating team.
Jordan's foreign minister presented a draft list of "terrorist" groups that the ISSG nations will agree to exclude from the talks.
This would include, but may not be limited to, the Islamic State group and the Al-Qaeda-linked Al-Nusra Front.
Kerry said that there were still sharp disagreements between the ISSG parties, some of which sponsor armed factions on the ground, as to which groups should be banned.
"There was a very rigorous discussion this morning, as you might imagine, about who might or might not qualify as a terrorist," he said.
He said it was "impossible to reach consensus" in the time available and that other countries would now help Jordan redraft the list in the weeks to come.
- 'Unrealistic' timetable -
Syria's main opposition group, the Syrian National Coalition, warned that hopes for a ceasefire by January 1 were unrealistic and demanded that Russia halt its air strikes as part of that truce.
Najib Ghadbian, the SNC's envoy to the United Nations, said opposition groups need "a month or so" to prepare for the political talks that would begin in tandem with a ceasefire.
And, on the eve of the talks, Assad warned in an interview with Dutch television that misguided efforts to bring about regime change would make the conflict "drag on."
Assad responded with sarcasm when asked whether he was comforted that Washington's stance on his departure had seemingly softened.
"I was packing my luggage. I had to leave, but now I can stay," he joked.