Iran is not among the first round of nations invited to attend Syria peace talks in Switzerland later this month, a UN spokesman has confirmed.

However, the UN Secretary-General Ban Ki-moon "is in favour of inviting Iran," spokesman Farhan Haq said on Monday.

Invitations to the talks are subject to approval by the initiating states, Russia and the US, which have not yet agreed on Iran's role in the talks.

Haq said the UN hopes that can be resolved at a January 13 meeting between US Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov.

Meanwhile, Tehran on Monday brushed aside a US offer of a sideline role in the so-called Geneva 2 talks, suggesting such an arrangement would not respect the country's "honour."

"Iran has always announced its readiness to participate [in the Syria talks] without preconditions," Foreign Ministry

spokeswoman Marzieh Afkham told the official IRNA news agency.

Afkham's comments came after Kerry suggested that Iran, Syria's main regional ally, could play a constructive role in finding a resolution to Syria's civil war, even without participating fully in the talks.

The US has objected to Iran's participation because Tehran has not publicly endorsed the call for a transnational government in Syria, and is backing militias propping up the regime.

"If Iran doesn't support that, it's difficult to see how they are going to be a ministerial partner in the process," Kerry said. "Now could they contribute from the sidelines? Are there ways for them, conceivably, to weigh in?... It may be that there are ways that that could happen," Kerry said.

About two dozen nations plan to send foreign ministers to Geneva 2, and Syria's warring factions have also been invited.

President Bashar al-Assad's government has said the president will not surrender power and may run again in elections due later this year, even as the death toll from Syria's three-year-old civil war surpasses 100,000.

The main opposition bloc, the Syrian National Coalition, has yet to decide on whether to participate in the talks.