Peace talks aimed at ending Syria's brutal conflict will likely be delayed by a few days past the scheduled start date on Monday, the United Nations said.

"It is likely the 25th may slip by a few days for practical reasons," said Jessy Chahine, a spokeswoman for UN Syria envoy Staffan de Mistura, who is scheduled to host the talks.

In an email to AFP on Thursday, Chahine stressed though that "we are still aiming for that date and we will in any event assess progress over the weekend."

Her comment contradicted a statement earlier Thursday from US Secretary of State John Kerry.

"It may be a day or two for invitations but there is not going to be a fundamental delay. The process will begin on the 25th and they will get together and see where we are," Kerry said on the sidelines of meetings at the World Economic Forum in Davos, Switzerland.

The latest bid to end Syria's nearly five-year war through indirect talks between the government and part of the opposition has long been scheduled to start in Geneva on Monday.

But a dispute over who should represent the Syrian opposition at the talks has cast a shadow of uncertainty over the start of the negotiations.

On Wednesday, the main coalition of opposition bodies, the so-called High Negotiations Committee, announced its own delegation to the talks.

But its decision to name a member of the Jaish al-Islam rebel group as its chief negotiator has drawn criticism both from some of its own members, and opposition figures excluded from the coalition.

The National Coordinating Committee for Democratic Change, a key opposition body still present in Damascus, said the choice was "not acceptable".

And Haytham Manna, an opposition figure who does not belong to the High Negotiations Committee, accused it of including "war criminals" in its delegation.

Damascus's delegation will be headed by the Syrian ambassador to the United Nations, Bashar al-Jaafari, and overseen by Deputy Foreign Minister Faisal al-Moqdad, the Syrian government source has said.