The main body representing Syrian opposition groups has set out its plan to end the war under which President al-Assad would step down after six months of talks.
A transitional body would then run the country for 18 months before elections were held.
It was immediately rejected by the Syrian government.
The High Negotiations Committee (HNC) – which is backed by Western powers – unveiled its plan in London, saying any US-Russian agreement would have to be similar.
“If what the Russians and the Americans agree upon is very much different from what the Syrians aspire to, then we shall not accept it. It is not possible to accept Bashar al-Assad – not for six months or a day or a one minute, in this transitional period,” said the HNC’s chief coordinator, Riad Hijab.
HNC (@SyrianHNC_en) September 7, 2016
Moscow and Washington back opposite sides in the five-and-a-half year conflict. Presidents Putin and Obama met recently at the G20 summit and discussed Syria but failed to reach an agreement.
President Assad has always refused to step down, and some observers say recent government successes on the battlefield have strengthened his position.