The US and Russian initiative, unanimously approved by the UN Security Council on Friday, foresees talks between the rebels and the regime and a rapid ceasefire, perhaps even next month.
But the plan was described as unrealistic by the Istanbul-based National Coalition, the main Syrian opposition grouping.
The resolution "undermines the outcome of the meetings of revolutionary forces in Riyadh and waters down previous UN resolutions concerning a political solution in Syria," coalition head Khaled Khoja said on Twitter.
The Security Council met 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.
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"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 said.
"And that means all the barrel bombs will stop, all the bombing, all the shooting, all the attacks on either side."
The resolution does not touch on this vital issue.
"We often hear the argument that without resolving the Assad question, it is impossible to truly coordinate in the fight against terrorism," Russian Foreign Minister Sergei Lavrov said.
"This is a dangerous logic, a dangerous approach," RIA Novosti news agency quoted him as saying.
Kerry, who has "agreed to disagree" with Moscow on Assad's fate, emphasised that victory over the Islamic State group hinges on a peaceful settlement in the civil war.
He said Assad had "lost the ability, the credibility" to unite and govern Syria.