The talks, scheduled to open in Geneva on Monday, are part of the biggest diplomatic push yet to resolve a civil war that in nearly five years has left more than 260,000 dead and forced millions from their homes.
World powers agreed last year in Vienna on an 18-month roadmap that would lead to the peace talks, a transitional government and then elections.
But the office of UN Syria envoy Staffan de Mistura, who is hosting the talks, said the January 25 deadline for the start of talks would probably be missed.
"We are still aiming for that date and we will in any event assess progress over the weekend," she said.
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US Secretary of State John Kerry said he was confident the talks would go ahead as planned.
"It may be a day or two for invitations but there is not going to be a fundamental delay," Kerry said on the sidelines of meetings at the World Economic Forum in Davos, Switzerland.
"The process will begin on the 25th and they will get together and see where we are," he said.
The main coalition of opposition bodies, the High Negotiations Committee, announced a delegation to the talks on Wednesday but its composition quickly came under fire.
The group named Mohamed Alloush, a political leader of the Saudi-backed Islamist armed group Jaish al-Islam, as its chief negotiator at the talks.
The National Coordinating Committee for Democratic Change, a key opposition body still present in Damascus, said it was "not acceptable" for the delegation's chief to come from the armed opposition.
The pro-regime Al-Watan daily called Alloush's appointment "a provocative step with the sole goal of thwarting any possible dialogue". Syria's ally Russia has said it considers Jaish al-Islam as a "terrorist" group.