Syria's government handed Russia a proposal for a local ceasefire and an exchange of prisoners, ahead of a peace conference due to start next week.

There was no immediate response from President Bashar al-Assad's opponents, whose attendance at the talks due to start on Wednesday in Switzerland remains in doubt.

Syrian Foreign Minister Walid al-Moualem said on a visit to Moscow that he had given Russian officials a plan for a truce in Aleppo and added that the government was ready to exchange lists of prisoners to be swapped.

Moscow and Washington have urged both parties to make concessions including ceasefires, aid access and prisoner exchanges to build confidence before the conference.

But there was little sign of coherent negotiating positions from the rebels who are fighting the government and each other.

Most of the rebel forces fighting inside Syria have dismissed the Geneva Two negotiations, with exiled opposition leaders currently meeting in Turkey to decide whether to take part.

The US issued a despairing 11th-hour appeal to the rebels to participate.

"The US urges a positive vote," Secretary of State John Kerry told rebel groups on Thursday.

"The Geneva peace conference is not the end but rather the beginning, the launch of a process that is the best opportunity for the opposition to achieve the goals of the Syrian people and the revolution."

But the opposition Syrian National Coalition has already seen 44 of its 120 members declare their hostility to joining the talks in Montreux, which they see as having little chance of forcing President Assad to step down.

Damascus has made clear that it rejects demands that President Assad makes way for a transitional leadership and has suggested the talks focus on co-operation against the terrorism of the Islamists that dominate the rebel front lines.

Russian Foreign Minister Sergei Lavrov criticised the Syrian opposition for its delay in agreeing to take part.

"It worries us very much that some kind of game is being played," he said.