The second round of Syria peace talks has got off to a stormy start against a backdrop of continued rescue of civilians from the beseiged city of Homs.
Members of Syria's government and opposition delegations blamed each other in Geneva on Monday for mortar attacks on convoys bringing aid to Homs.
Reporting from the Swiss city, James Bays, Al Jazeera's diplomatic editor, said the atmosphere at the peace talks was "the worst it has been at any point during this process".
The humanitarian exit operation is part of a deal brokered by the UN between President Bashar al-Assad's government and the Syrian opposition after months of negotiations.
But a ceasefire underpinning the effort proved fragile, with the first aid convoy coming under attack on Saturday and mortar rounds hitting a rebel-held district on Sunday, killing five people.
Activists have blamed pro-regime factions in neighbourhoods bordering the rebel-held districts for the attacks, while Syrian state television said "armed terrorist groups" had fired during the UN operation.
The UN, for its part, said the three-day evacuation mission would be extended until Wednesday night.
The announcement was welcomed by Valerie Amos, the UN humanitarian chief.
"I hope this will allow us to evacuate yet more civilians and deliver much needed additional supplies," she said in a statement which reported that more than 800 people have been helped out of Homs since late on Friday.
"The protection of civilians caught up in this horrendous conflict in Syria is the greatest priority for UN agencies and humanitarian partners."
The civilians who left were among an estimated 3,000 people trapped since June 2012 in parts of the Old City held by rebels and under attack by Assad's forces.
The Syrian army has staged huge offensives, with near-daily bombardments killing thousands.
Recriminations in Geneva
Monday's talks in Geneva saw Lakhdar Brahimi, the joint UN-Arab League mediator, meet separately the government and opposition teams, in the hope that keeping them apart at first might help achieve more than during a virtually fruitless first round last month.
The opposition delegation gave warning that it would not return for a third round if no progress was made in the so-called Geneva 2 talks.
"If there is no progress at all, I think it would be a waste of time to think about a third round," Louay Safi, the National Coalition spokesman, said, adding that the delegation had raised the issue with Brahimi.
The negotiators were set to sit down for talks together on Tuesday, both sides said.
"Tomorrow, at 10am (0900 GMT), there will be a joint session," Badr Jamous, the National Coalition's secretary-general, told AFP news agency.
"We are ready to confront the regime at any time and anywhere."
A source in the government delegation confirmed that there would be a joint meeting on Tuesday.
The opposition insists that the only way to put an end to the nearly three years of civil war is to put in place a transitional government - without Assad.
The Syrian government, however, insists that Assad's future is not up for negotiation.
Instead, the regime wants the talks to focus on stopping violence and "terrorism" - its term for the revolt, which it says has been spearheaded by foreign fighters and Gulf Arab money.
With the talks at an apparent standstill, Russia proposed on Monday that it and the US hold a collective meeting with the UN and the two sides to try to move things forward.
US, which backs the opposition, and Russia, a key ally of Syria, initiated the Geneva 2 talks and pushed for eight months to get the parties to the negotiating table.
Meanwhile, UN diplomats in New York resumed on Monday the task of trying to persuade Russia to back a new humanitarian resolution, only to see the effort firmly rejected by Russia's ambassador.
A draft text of the non-binding resolution, seen by AFP news agency, prepared by Australia, Jordan and Luxembourg "demands that all parties, in particular the Syrian authorities, immediately end the sieges of the Old City of Homs", as well as in Aleppo, Damascus and other cities.
Such steps are needed "to allow the delivery of humanitarian assistance, including medical assistance," it said.
Monday's meeting included permanent Security Council members Britain, the US and France.
However, Russia's envoy to the UN, Vitaly Churkin, did not attend, nor did China's representative.