Syria's opposition will decide this week whether to attend forthcoming peace talks in Geneva and has complained to the United Nations that Russian air strikes have carried on despite a truce, opposition coordinator Riad Hijab says.
Syrian government forces, backed by Russia's air force, Iranian troops and Iraqi militias, have continually breached the temporary ceasefire and used barrel bombs and toxic gas, he told reporters on a conference call on Monday.
Syria's armed forces have always denied using barrel bombs or chemical weapons.
"Just a few minutes before we came on this conference there was a massacre committed by the air forces of the Russians and the regime in Abu Dhuhour," he said, referring to a settlement in eastern Idlib governorate.
Tens of people had been killed and tens wounded, he said.
The opposition High Negotiations Committee (HNC) would consult military commanders and other leaders about whether to attend the talks, he said.
"It will be before the end of this week, there will be a clear decision about this," Hijab said through a translator.
Hijab said he had sent a letter to UN Secretary General Ban Ki-moon to complain that a map of armed groups' positions, published by the Russian Defence Ministry and which would legitimise air strikes in certain places, was inaccurate.
"If it is being taken seriously it would represent a clear and flagrant violation of the cessation of hostilities agreement," he said.
Russia has said its air strikes are only targeting banned groups that are not part of the ceasefire deal, such as the Nusra Front. Hijab said Nusra's positions were close to but "not interlinked" with opposition positions.
If the talks go ahead, the formation of a transitional governing body will be top of the agenda, Hijab said. He said that President Bashar al-Assad must leave Syria as soon as such a body is formed and face international justice for war crimes.
The UN resolutions governing the peace process say there must be free and fair elections, supervised by the United Nations, within 18 months, without specifying anything about Assad. But Hijab insisted Assad could not contest the election.
