The UN said on Thursday that there was no postponement of Syria peace talks due to start Friday in Geneva, despite uncertainty around whether the main opposition umbrella group would attend.

There is "no ­postponement from our side," Khawla ­Mattar, spokeswoman for the talks, said as the High Negotiations Committee, formed last month in an effort to unite ­Syria's fractious opposition, said in ­Riyadh it was waiting for answers from the UN before agreeing to attend.

The UN Syria mediator has responded to opposition demands for an end to airstrikes and blockades on behalf of UN Secretary-General Ban Ki-moon, a UN spokesman said on Thursday.

"Staffan de Mistura's response to Riad Hijab, both by phone yesterday and in ­writing last night, constitutes the response also on behalf of the Secretary-General," UN spokesman Stephane Dujarric said.

Riad Hijab is the coordinator of the opposition Committee. Talks to end Syria's civil war had risked being delayed for the second time this week as the opposition stuck to its demands on Thursday.

Syrian opposition members met for a third day Thursday to decide whether to attend UN peace talks, with less than 24 hours before the negotiations were due to open in Geneva.

Western diplomats have piled pressure on the opposition to take part in the negotiations, part of the biggest push yet to resolve Syria's nearly five-year civil war.

But after two days of meeting in the Riyadh, the ­Committee had yet to agree to participate by press time.

The Committee had asked for "clarifications" after the UN issued invitations to other opposition figures and is demanding assurances from the international community that it will move to end government attacks on civilians and allow humanitarian aid.

Salem al-Meslet, a Committee spokesman, said UN Syria envoy ­De Mistura had already assured the opposition that two of the resolution's articles - calling for immediate access for humanitarian aid and an end to attacks on civilians - were non-negotiable.

Security Council members "must take their responsibilities and commit to applying resolution 2254. We are waiting for an answer," Meslet said.

The talks are part of a UN-backed plan, agreed by top diplomats last year in Vienna, that envisages negotiations followed by a transitional government, a new constitution, and elections within 18 months.

The roadmap is the most ambitious plan yet to end the conflict which has killed more than 260,000 people and forced millions from their homes.