Site of airstrike in Syria
Site of airstrike in SyriaReuterrs

The United Nations said Thursday that crucial talks on ending the Syria conflict would likely be delayed by a few days, as a dispute over who will represent the opposition intensified.

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.

"It is likely the 25th may slip by a few days for practical reasons," Jessy Chahine, a spokeswoman for de Mistura, told AFP in an email.

"We are still aiming for that date and we will in any event assess progress over the weekend," she said.

U.S. Secretary of State John Kerry had earlier 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.

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.

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.

"This sends the wrong political message to the Syrian people," it said.

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.

But a French diplomatic source said the opposition would lack credibility if it was "not represented by forces on the ground" such as Jaish al-Islam.

There is also disagreement, however, over whether the opposition will be represented by one or two delegations.

The High Negotiations Committee insists it must be the sole opposition representative, but other opposition figures and Syria's Kurds have been excluded from its ranks and also want to participate in the talks.

Al-Watan reported that Russia was proposing a joint opposition delegation.

A Syrian government source meanwhile said earlier Thursday that the country's envoy to the United Nations, Bashar al-Jaafari, would serve as the regime's chief negotiator at the talks.

Jaafari had the same role in a previous round of peace talks in Geneva in 2014.

AFP contributed to this report.