Plans to hold the first negotiations to end the civil war in Syria for two years are in doubt after the opposition said it would not show up unless the United Nations responded to demands for a halt to attacks on civilian areas.
The Syrian government has already agreed to join the talks that UN envoy Staffan de Mistura hopes to convene in an indirect format in Geneva on Friday with the aim of ending the five-year-old war that has killed 250,000 people.
The United States has urged Syrian opposition groups to attend.
"Factions of the opposition have an historic opportunity to go to Geneva and propose serious, practical ways to implement a ceasefire, humanitarian access and other confidence-building measures, and they should do so without preconditions," US State Department spokesman Mark Toner said.
Preparations have been beset by difficulties, including a dispute over who should be invited to negotiate with Syrian President Bashar al-Assad's government as it claws back territory with help from Russia and Iran.
Kurds, who control a swathe of northern Syria, were not invited and predicted the talks would fail.
A Saudi-backed opposition council that groups armed and political opponents of Mr Assad broke up a second day of meetings in the Saudi capital, Riyadh, saying it was waiting for a response from the UN to demands before it decided whether to attend.
While it has expressed support for a political solution and talks, the opposition High Negotiations Committee says attacks on civilian areas must stop before any negotiations.
In a letter to UN Secretary General Ban Ki-moon, it also called for the lifting of sieges on blockaded areas among other steps outlined by the UN Security Council in a resolution passed last month.
"We are waiting for the response of de Mistura first, and then Secretary General Ban Ki-moon, which is the most important... If it is positive maybe there will be an agreement to go," Asaad al-Zoubi, an HNC member, told Saudi TV channel Al Ekhbariya.
Diplomacy has so far failed to resolve a conflict that has forced millions from their homes, creating a refugee crisis in neighbouring states and Europe.
With the war raging unabated, the latest diplomatic effort has been overshadowed by increased tension between Saudi Arabia and Iran.
The Syrian government, aided by Russian air strikes and allied militia including Iranian forces, is gaining ground against rebels in western Syria, this week capturing the town of Sheikh Maskin near the Jordanian border.
Russian air strikes that began on 30 September have tilted the war Mr Assad's way after major setbacks earlier in 2015 brought rebels close to coastal areas that form the heartland of Mr Assad's Alawite sect and are of great importance to the state he leads.
While the Saudi-backed HNC includes powerful rebel factions fighting Mr Assad in western Syria, Russia has been demanding wider participation to include Syrian Kurds who control wide areas of northern and northeastern Syria.
The Syrian Kurdish PYD party, which is affiliated with the Kurdish YPG militia, was however excluded from the invitation list in line with the wishes of Turkey, a major sponsor of the rebellion which views the PYD as a terrorist group.
Kurdish leaders forecast talks will fail
The PYD's representative in France, Khaled Eissa, who had been on a list of possible delegates proposed by Russia, blamed regional and international powers, in particular Turkey, for blocking the Kurds and forecast the talks would fail.
"You can't neglect a force that controls an area three times the size of Lebanon," he said. "We will not respect any decision taken without our participation."
Russian Deputy Foreign Minister Gennady Gatilov said the PYD could join the talks at a later stage.
Claims Syria ignored most requests to deliver aid
Meanwhile, the UN's aid chief has said the Syrian government ignored most requests to deliver humanitarian aid in 2015.
The requests involved some 4.6 million people in hard-to-reach and besieged areas, with only 620,000 receiving help.
Stephen O Brien told the UN Security Council that last year the UN made 113 requests to the Syrian government for approval of inter-agency aid convoys, but only 10% were able to deliver assistance.
Another 10% were approved in principle by the Syrian government, but could not proceed due to a lack of final approval, insecurity or no deal on safe passage, while the UN put 3% on hold due to insecurity.
O'Brien said the remaining 75% of requests went unanswered.
