Talks between Syria's warring sides entered their fifth day in Geneva on Wednesday with no signs of progress on a transfer of power or on providing desperately needed humanitarian relief.
Delegations from President Bashar al-Assad's regime and the opposition National Coalition sat down from around 11:00 am (1000 GMT), the UN said.
The two sides have been unable to even begin discussing political questions, let alone the core issue of creating a transitional governing body.
There has been disappointment over the failure to agree any humanitarian measures, especially for besieged residents in the central city of Homs, where UN trucks are waiting for access to deliver food and medical aid.
Tuesday's talks fell apart after the regime delegation urged participants to adopt a statement condemning Washington for supporting "terrorists" in Syria by backing rebel groups.
The opposition said it had presented its "vision" of the way forward but that the regime had refused to discuss it.
UN mediator Lakhdar Brahimi said he had decided to cancel Tuesday afternoon talks and reconvene the parties for "what I hope will be a better session" on Wednesday morning.
"Nobody is walking out, nobody is running away," Brahimi told reporters on Tuesday. "We have not achieved any breakthrough, but we are still at it, and this is good enough as far as I'm concerned."
A source in the opposition delegation told AFP the regime had been asked to say on Wednesday how it views the Geneva communique -- the statement on forming a transitional government that was issued after talks among global powers here in 2012.
"Brahimi yesterday was very clear: the regime delegation must present its agenda and its vision of Geneva I," the source said.
Neither side walking away
The two sides have been brought together in the biggest diplomatic push yet to end a civil war that has left more than 130,000 dead and forced millions from their homes.
Expectations are low for a breakthrough, especially after the talks hit an impasse Monday when the two sides failed to agree on even the basic principles of political talks.
Despite their frustration, each side vowed it would not be the first to walk away from the talks, which are expected to last until Friday.
In the only tangible promise to emerge from the meetings so far, Brahimi said Sunday the regime had agreed to allow women and children safe passage from besieged rebel-held areas of Homs.
But there has been no movement since, on either an evacuation or Brahimi's hope that aid convoys will be allowed in the areas.
The Old City of Homs has been under siege since June 2012 after rebels there rose against the regime, with an estimated 500 families living with near-daily shelling and the barest of supplies.
UN bodies and the International Committee of the Red Cross have said they are on standby with aid but are waiting for approval to move in.
"The convoy is ready and still waiting to enter. The authorisation has not been given yet," Brahimi said Tuesday. "We haven't given up on that."
It took months of pressure from Washington, which backs the opposition, and Moscow, Assad's key international ally and arms supplier, to bring the two sides together.
But in his annual State of the Union address, President Barack Obama made little mention of Syria, saying only that his administration would continue to work for a future "the Syrian people deserve -- a future free of dictatorship, terror and fear".