A fresh round of peace talks between Syria's government and opposition has got off to a rough start, with few signs of progress, UN-Arab League mediator Lakhdar Brahimi said Tuesday.

"Today I don't have much to tell you except that the beginning of this week is as laborious as it was," Brahimi told a news conference in Geneva after a joint meeting of the two sides. "We are not making much progress."

The frustration was also conveyed by Syria's government delegation, who said no agenda had been agreed for the ongoing peace talks and blamed the opposition's refusal to discuss the issue of "terrorism."

"Another lost day because the representatives of the coalition insisted that terrorism in Syria does not exist and did not want to discuss it," said Deputy Foreign Minister Faisal Mekdad after the second day of talks.

"This should be the priority for any Syrian ... After that we are ready to discuss anything," Mekdad said.

The second round of talks got off to a shaky start Monday, with Brahimi shuttling between the delegations in the hope that keeping them apart at first might put them in a more conciliatory state of mind.

An eight-day session last month achieved little beyond getting the foes into the same room for the first time since Syria's civil war erupted in March 2011.

There has been scant sign that the current round, expected to last until Friday, will make progress toward ending the conflict.

The two sides spent Monday trading blame for escalating violence and for difficulties evacuating civilians and getting aid to opposition-held districts of the city of Homs, under government siege since 2012.

Neither appeared prepared to budge an inch on their negotiating positions.

The opposition warned Brahimi that it will not return for more talks if there is no progress this week.

"We're not going to run away" if there is a glimmer of hope, but if there is no progress "let us not pretend we are doing something," spokesman Louay Safi said Monday.

Safi said that in such a case it would be "more honest to say we have failed," although he acknowledged the only alternative was to continue fighting.

The opposition delegation this time includes seven representatives of fighters on the ground, the spokesman said.

With the talks at an apparent standstill, Russia on Monday proposed that Moscow and Washington hold a collective meeting with the UN and the two sides to break the deadlock.