Even as top diplomats vowed to keep trying, President Barack Obama expressed scepticism that an unlikely alliance between rivals would yield the breakthrough needed to end the 5-year-old civil war.

Still, as Obama and Russian President Vladimir Putin attended an economic summit, the leaders were under pressure to push the negotiations beyond the sticking points that have thus far prevented a deal.

White House spokesman Ned Price said Obama and Putin were huddling on the sidelines of the Group of 20 economic summit, with details of their informal meeting expected to be released later on Monday. A second day of talks in China between Secretary of State John Kerry and his Russian counterpart, Sergey Lavrov, ended Monday without a deal to announce.

The two diplomats met for an hour but emerged still at odds on certain issues, said a senior State Department official, who wasn't authorised to discuss the talks by name and requested anonymity. The talks culminated a several weeks of searching for a cease-fire between Syrian President Bashar Assad's government and moderate rebels that would expand access for hundreds of thousands of civilians caught in the crossfire.

The strategy has hinged on an unlikely US-Russian militarily partnership against extremist groups operating in Syria. But beyond the Islamic State group and al-Qaida, the two powers have conflicting views about who fits in that category - as well as a deep and mutual distrust that the other party will hold up its end of the bargain.

"We're not there yet," Obama told reporters yesterday. "It's premature for us to say that there is a clear path forward, but there is the possibility at least for us to make some progress on that front." Obama's wariness was less apparent among his State Department negotiators, who had been so hopeful a deal could come together while world leaders gathered in China that they scheduled a press conference and announced plans to brief reporters on the pact.