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.

US Secretary of State John Kerry said he was confident the talks would go ahead as planned.

Diplomats appeared to be scrambling to resolve the key question of who will represent Syria's fractious opposition at the talks with President Bashar al-Assad's regime.

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