The US said Sunday it had not yet struck a hoped-for deal with Russia on stemming the violence in Syria's brutal civil war, blaming Moscow for backtracking on issues it thought were settled.

President Barack Obama said earlier that the two sides were ‘working around the clock’ on the sidelines of a summit in China, but that it was ‘a very complicated piece of business.’

The State Department said a deal was close and could be announced by Secretary of State John Kerry and his Russian counterpart Sergei Lavrov, but hours later admitted defeat for now.

‘Russians walked back on some of the areas we thought we were agreed on, so we are going back to capitals to consult,’ a senior State Department official said.

Kerry and Lavrov will meet again on Monday in Hangzhou, where G20 leaders are gathered, he added.

The Secretary of State told reporters: ‘We're going to review some ideas tonight, a couple things on these couple of tough issues, and come back together and see where we are.

‘We're not going to rush,’ he said, stressing the importance of reaching a deal that was able ‘to try to get the job done’.

There remained ‘a couple of tough issues’ to deal with, he said, declining to give details.

Moscow and Washington support opposite sides in the Syrian conflict, which erupted in March 2011 after President Bashar al-Assad unleashed a brutal crackdown against a pro-democracy revolt.

Successive rounds of international negotiations have failed to end a five-year conflict that has left more than 290,000 people dead and forced millions to flee, a key contributor to migrant flows into Europe.

Russia is one of Assad's most important international backers while the US supports Syria's main opposition alliance and some rebels, with other countries and forces also involved.

‘Trying to corral all of those different forces into a coherent structure for negotiations is difficult,’ Obama said Sunday. ‘But our conversations with the Russians are key.’

- Fears for Aleppo -

The US and Russia co-chair a UN-backed humanitarian taskforce for Syria, which has been struggling to ensure access for desperately needed aid across the country.

The battered second city Aleppo, which is divided between government and opposition control but surrounded by loyalist forces, has emerged as a major concern with urgent calls for a ceasefire to alleviate a humanitarian catastrophe.

The talks in Hangzhou are the latest round of diplomacy on Syria, after marathon negotiations between Kerry and Lavrov in Geneva last week failed to yield a final deal.

Kerry then listed two main priorities to ensure any new ceasefire holds: responding to violations by the Damascus regime and checking the rising influence of the former al-Nusra Front.

That group has renamed itself Fateh al-Sham Front after renouncing its status as al-Qaeda's Syrian affiliate, but Kerry stressed that ‘Nusra is al-Qaeda, and no name change by Nusra hides what Nusra really is and what it tries to do’.

Earlier truces have rapidly deteriorated, and Obama warned Sunday that the US was approaching the talks ‘with some scepticism’.

‘But it is worth trying,’ he said.

‘To the extent that there are children and women and innocent civilians who can get food and medical supplies and get some relief from the constant terror of bombings, that's worth the effort.’