Veteran negotiator Lakhdar Brahimi, who wants to revive efforts to bring the warring parties to the negotiating table, called on the two powers to build on the momentum from their deal to dismantle Syria's chemical weapons.
"What matters now is that this rapprochement between Russia and the United States is confirmed, lasts, and that together these two leading countries mobilise other leading countries so that we finally move towards a political solution," he said in an interview with Swiss public broadcaster RTS.
"But to be realistic, they're not going to be before October, and probably not in early October," he said.
"The problem isn't getting a conference started, the problem is to be sure that there's real, solid political will from the players who count."
More From This Section
A peace conference was first proposed by the United States and Russia in May but momentum toward holding the event was lost as Washington and Syria's staunch ally Moscow bickered over the conflict.
Brahimi said he would discuss plans with US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov on September 28 during a UN General Assembly session in New York.
The US-Russia deal aimed at dismantling Syria's chemical weapons that was reached last Saturday in Geneva has raised hopes that a conference can be finally held.
The so-called Geneva II talks are meant to be based on an agreement in the Swiss city in June 2012, when world powers said Syria needed a transition government.
But the warring sides failed to agree on whether Assad could play a role in forming it, or whether his inner circle could serve on the interim team.
The conflict erupted after a crackdown on protests against the Assad regime in March 2011.
It morphed into a sectarian war which has claimed over 100,000 lives, driven more than two million people from the country, and displaced millions more within Syria.
"There is no military solution to this conflict. There is no side which can win the war," said Brahimi.