The main Syrian opposition's decision not to attend formal peace talks is a "mistake" as a political settlement can only come through negotiations, but it does not signify a breakdown in the effort, a senior Russian envoy said on Tuesday.

Alexei Borodavkin, Russia's ambassador to the United Nations in Geneva, also told Reuters that the opposition High Negotiations Committee (HNC) lacked a political vision for a transition period and seemed incapable of striking any deal.

"The announcement by the HNC of its decision to suspend their participation in proximity talks here on the Syrian settlement was a mistake," Borodavkin said in an interview at Russia's diplomatic mission.

"We are convinced that there is no alternative for a political settlement through negotiations. We also think that they should be transformed into direct negotiations among the Syrian parties concerned," he said.

The peace talks appeared doomed after air strikes killed about 40 people in a market in rebel territory on Tuesday, with the HNC saying a truce was finished and it would keep away from negotiations indefinitely.

"Our assessment is that the HNC suspended their participation in the talks not because of these reasons which are absolutely artificial and not true actually. But according to our information it is just because they do not have a clear political position. The only approach they have is 'Assad should resign'. Nothing but that. But this is not enough," Borodavkin said.

U.N. mediator Staffan de Mistura has been conducting separate talks with the warring sides, following a truce that entered into force on Feb. 27.

"Staffan de Mistura is trying to focus them on the transitional period, the transitional institutions to be established and how they should work, what would be the composition. And there is nothing, there is nothing on the part of the HNC," Borodavkin said.

"They suspended participation not because they have real reasons in the humanitarian sphere or be it the cessation of hostilities, but as we see it they just do not have a position for the negotiations. They are not capable to discuss things and even less so make agreements."

The government of Syrian President Bashar al-Assad, which Russia has backed throughout the five-year conflict, "is ready to discuss the political transition", he said.

The Syrian government's chief negotiator Bashar Ja'afari said on Tuesday his team was pushing for an expanded government led by Assad at U.N.-led peace talks, and Assad's own future was not up for negotiation.

Borodavkin, asked about whether Assad's future was not on the table, replied: "We think it is for the Syrian people to elect its leader. Full stop, period."

"It's not for the opposition delegation or for the government delegation to try to make a decision on such important issues. It's for the Syrian people themselves. This should be decided not during the negotiations but at the polling stations."