Kerry told reporters that he might meet Russian Foreign Minister Sergei Lavrov in Geneva, but indicated detailed talks were more likely to take place on the sidelines of a gathering of Southeast Asian nations in Laos on July 25-26.

Kerry defended the US proposal, which has been greeted with deep skepticism by top American military and intelligence officials.

"The president of the United States has authorized and ordered this track," he said. "It is the president’s desire to test whether or not the Russians are prepared to do what they said during our negotiations in Moscow that they will do."

"I don't comment on internal negotiations" within the US government, Kerry said. "The place for these arguments to be argued is in the (White House) Situation Room with the president of the United States, and he has made a decision."

The proposed agreement - which Kerry discussed in Moscow with Russian President Vladimir Putin last week - would require the Syrian government to cease attacks on US-backed Syrian opposition fighters. In return, Russia would receive US intelligence help to target militant groups in Syria such as Islamic State and al Qaeda affiliate Nusrah Front.

"We’re going to test this very carefully based not on trust, based on specific steps," Kerry said of the diplomacy with Moscow. "So far, it is showing a modicum of promise which, hopefully, we can complete."