The UN's Syria envoy said he was "very, very sorry" Saturday after Geneva peace talks broke off without result, throwing the future of the negotiations to end the bloody conflict into doubt.

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, with no date set for a third round.

"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?"

Britain's Foreign Secretary William Hague decried the situation as "a serious setback in the search for peace in Syria."

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.

Brahimi noted that the two sides had at least finally agreed on an agenda for possible future talks.

"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.

Washington has voiced deep frustration at the stalemate and chastised Moscow, its co-sponsor of the so-called Geneva II talks, for not doing enough to push its ally to engage "seriously" in the process.

US President Barack Obama vowed 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 without specifying what such steps might be.

French Foreign Minister Laurent Fabius also blamed the failure of the talks squarely on Damascus.

The regime "blocked any progress on establishing a transition government and stepped up violence and acts of terror against the civilian population," Fabius said.

AFP