Veteran negotiator Lakhdar Brahimi, who wants to revive efforts to hold a peace conference in Geneva aimed at bringing the warring parties together, called on the two powers to build on the momentum from a deal on 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.
Brahimi said he would like to see negotiations begin in Geneva "the day after tomorrow".
"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."
The conference was first proposed by the United States and Russia in May but momentum towards holding the event was lost as Washington and Syria's staunch ally Moscow bickered over the conflict.
It is expected to be discussed when Brahimi meets US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov on September 28th on the sidelines of a UN General Assembly session in New York.
Washington and Moscow were deadlocked over how to deal with President Bashar al-Assad until a deadly August 21st chemical attack provided the impetus for agreement.
The so-called Geneva II peace talks are meant to be based on the results of talks in the Swiss city in June 2012, when world powers agreed that 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 closest representatives could serve on the interim team.
That dispute, coupled with an inability to halt the fighting on the ground, meant the 2012 plan was never implemented.
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 refugees from the country, and displaced millions more within Syria.
"There is no military solution to this conflict," said Brahimi.
"There is no side which can win the war," .