A major Syrian opposition grouping was still awaiting clarifications from the United Nations Thursday as it met for a third day on participation in UN-brokered peace talks to begin within 24 hours.

The Saudi-based High Negotiations Committee insists it must be the sole opposition delegation at the talks and has requested more information after the United Nations issued invitations to other regime opponents.

The HNC had still not received a reply from UN Syria envoy Staffan de Mistura, a day before talks are to start in Geneva.

The Geneva talks have been delayed since Monday over who will represent the opposition.

"Not yet," Salem al-Meslet, a Committee spokesman, told AFP when asked if de Mistura had responded.

"We are going now to start the meeting," he said at a Riyadh hotel, giving no further comment.

The HNC excludes Syria's most powerful Kurdish organisation, the Democratic Union Party (PYD), and a range of other opposition figures, seen as close to the Damascus regime and its ally, Russia.

In a written statement on Tuesday, HNC coordinator Riad Hijab referred to the Syrian "regime's attempts to jeopardise the political process through questioning the credibility of the opposition delegation" and trying "to insert some controversial names into the delegation formation".

Since Monday, the Committee has also been seeking UN clarification on the agenda of the talks, particularly that the international community would address humanitarian issues.

In his statement, Hijab said a UN Security Council resolution calling for an end to sieges of certain areas in Syria and the delivery of humanitarian aid "should not be ignored".

"It would not be possible to hold serious negotiations while war crimes are being committed against Syrians by foreign parties," he said.

Moscow in September began air strikes in support of Syrian President Bashar al-Assad, and has criticised Hijab's coalition as unrepresentative.

Following months of effort, Riyadh in December brought together about 100 representatives of the main Syrian political opposition and armed factions for unprecedented talks.

They agreed to negotiate with the regime but insisted Assad step down at the start of any political transition.