Syria’s opposition insists it will not attend talks on political solution to the war in the country despite the UN announcing they will begin on Friday.
Opposition leaders say representatives will not travel to Geneva unless steps are taken to alleviate the plight of civilians under siege and bombardment.
But UN envoy Staffan de Mistura plans to proceed with indirect “proximity” talks with the Syrian government.
More than 250,000 people have died in almost five years of war in Syria.
Eleven million others have been forced from their homes as forces loyal to President Bashar al-Assad and those opposed to his rule battle each other – as well as jihadist militants from so-called Islamic State (IS).
The last talks aimed at ending the conflict broke down in February 2014 after only two rounds, with the UN blaming the government for refusing to discuss an opposition demand for Mr Assad to step down.
Despite little sign of a change in that stance, the rise of IS prompted the US and Russia to step up their efforts to get the warring parties back to the negotiating table.
The proximity talks are expected to last for six months, with delegations sitting in separate rooms and UN officials shuttling between them.
The immediate priorities are a broad ceasefire, humanitarian aid deliveries and halting the threat posed by IS. But the ultimate aim is a peace settlement that includes a transitional period ending with elections, in line with a UN Security Council resolution approved last month.
Mr Assad has agreed to send a delegation to Geneva for Friday. But the Saudi-backed High Negotiations Committee, which was formed after a conference of political and armed factions last month, has declared that its delegation will “certainly” not be there.
After a meeting in Riyadh on Thursday, the committee complained that it had not received convincing answers to its demands for an end to air strikes and blockades by government forces.
The head of the committee, Riad Hijab, told al-Arabiya TV: “Tomorrow, we will not be in Geneva. We could go there but we will not enter the negotiating room if our demands are not met.”
Earlier, the UN envoy sent a video message to the Syrian people in which he warned that talks “cannot fail”.
“Now we need to hear your voice to everyone who is coming to this conference, and saying this conference must be an opportunity not to be missed,” he said.