GENEVA - The UN's Syria envoy said he was "very, very sorry" Saturday as peace talks in Geneva broke off with no progress made and no date set for a third round.

Just weeks after the warring parties sat down for the first time to seek a political settlement to the three-year conflict, a second round ended in acrimony.

"I'm very, very sorry," UN mediator Lakhdar Brahimi told reporters in Geneva as he announced the break-off in talks. "I think it is better that every side goes back and reflects, and takes their responsibility: do they want this process to continue or not?"  With no guarantee the parties will return to the negotiating table, the death toll continued to mount in Syria where more than 140,000 have died and millions have been driven from their homes.

A monitoring group said this week more than 5,000 people had been killed since the talks began on January 22.

And the UN raised the alarm over Syrian air raids in the Qalamun mountains near the Lebanese border, as thousands fled the opposition-held town of Yabrud amid fears of a ground assault.

In Geneva, the rivals have seemed to agree on only one thing this week: that the negotiations were stalled. "The regime is not serious... I'm very sorry to say there is nothing positive we can take from this," opposition spokesman Louay Safi told reporters after the talks. The head of the regime's negotiating team Bashar Jaafari described the other side as "amateurs", blaming the opposition's backer the United States for "trying its best to undermine the whole process." 'I very much hope there will be a third round' Brahimi noted that the two sides at least had agreed on an agenda for future talks - if they take place - something they failed to do throughout the week. "I very much hope there will be a third round," Brahimi said. The opposition says the focus must be on creating a transitional government, without President Bashar al-Assad. The regime representatives have insisted Assad's position is non-negotiable and refused to discuss anything beyond the "terrorism" it blames on its opponents and their foreign backers.

Brahimi said if the sides returned, they would discuss violence and terrorism first, then the transitional governing body (TGB), followed by national institutions and finally national reconciliation and national debate. However, he said the regime side baulked at his suggestion they spend one day on violence and the next on political transition "which raises the suspicion of the opposition that the government doesn't want to discuss TGB at all."

Safi of the opposition confirmed that suspicion, stressing that "a third round without talking about transition would be a waste of time." Jaafari denied his delegation had blocked the process, accusing the opposition of "misleading public opinion" and insisting that the regime "will be back." "We don't have an impasse," he said.

US Obama vowed on Friday to push the regime harder. "There will be some intermediate steps that we can take to apply more pressure to the Assad regime," he said after talks with Jordanian King Abdullah II in California, but did not specify what such steps might be. The ongoing evacuation of civilians from besieged rebel-held areas of Homs - agreed upon in the first round seen as the only tangible result so far of the Geneva II talks - has been hailed as a relative success.

Brahimi said Saturday the Homs agreement may have sparked hope in Syria that the talks could mark the beginning of the end "of this horrible crisis they are in."

"I apologise to them that these two rounds have not helped them very much," he said.