Syrian President Bashar al-Assad said in Damascus on Sunday he has no intention of giving up power and the issue is not up for discussion, Russian agency Interfax reported.

"If we wanted to give up, we would have done so at the very beginning. We are on guard for our country. This issue is not up for discussion," he was reported as telling visiting Russian parliamentarians, days ahead of an internationally sponsored peace conference on Syria.

The comments underscored differences between participants ahead of the talks in Montreux, Switzerland on Wednesday, seen as the most serious global effort yet to end Syria's three-year conflict.

The United Nations hopes the talks will bring about a political transition in the country, and US Secretary of State John Kerry said last week that Syria's future had no place for Assad.

Syrian National Coalition, Syria's main political opposition group in exile, agreed on Saturday to attend the talks and said three rebel fighting forces also wanted to take part.

National Coalition spokesman Louay Safi told Reuters the Soldiers of the Levant, the Syrian Revolutionaries Front and the Mujahideen Army all wanted "to have some representation within the delegation" at the talks.

Rebel brigades had previously rejected the conference - demanding the removal of Assad before talks. Their support is seen as critical if any deals have any chance of being rolled out.

A fourth fighting group, the Islamic Front - thought to be bigger than the other three combined - was still deciding whether to attend, Safi added.

Al Qaeda-linked rebels, increasingly involved in the fighting, have shown no interest in a political process.

Western powers had pressed the opposition to commit to the talks and on Saturday France welcomed the coalition's decision, vowing to make sure the discussions ended up setting up a transitional Syrian government with full executive powers.