With prospects for peace elusive, the broad-based opposition High Negotiations Committee (HNC) - holding talks with its foreign allies in London - proposed a six-month negotiating phase with the regime.
Boris Johnson has backed proposals by Syrian opposition figures to remove President Bashar Assad and establish a transitional government.
A transitional body would then run the country for 18 months before elections were held.
U.S. officials are particularly concerned by the deteriorating conditions in and around Syria's second city, Aleppo, where rebel-held eastern areas are once again under siege after government forces advanced over the weekend.
Following negotiations, Al-Assad will be expected to relinquish his grip on power before a transitionary phase lasting a year and a half that will see the drafting of a new constitution, the formation of a joint military committee and the establishment of a non-partisan governing council that will govern Syria while national reconciliation dialogue takes place. The Russians and Americans know that.
They also said they would convey to the British people the great deal of unbearable pain and suffering the Syrian people have to put up with because of crimes committed by terrorist groups inside the country.
That would allow the HNC, the "broadest-based" opposition group in Syria, to bring forward its plan for a post-Assad Syria that is "democratic and pluralistic" but which does not want to "sweep away all the existing structures of the state".
Writing in The Times newspaper, British Foreign Secretary Boris Johnson, the host of Wednesday's London talks, urged Russian Federation to cease its support for the Syrian president.
France's Ayrault said the UN Security Council should condemn the Assad regime and IS "for their continued use of chemical weapons".
But, he said, they were still keeping Assad in power because of concerns over what happens after he leaves.
U.S. Defense Secretary Ash Carter said U.S. diplomats were trying to persuade Russian Federation to take steps toward a true ceasefire in Syria and to nudge Damascus toward a political transition but news out of Syria was "not encouraging".
However, efforts are still going on behind the scenes and Saudi Arabia's Foreign Minister Adel al-Jubeir said in London on Tuesday that an agreement was possible within 24 hours.