HANGZHOU, China: Top diplomats from the United States and Russia on Monday failed to reach a deal to ease fighting in Syria, with a senior State Department official saying differences remained.

The official said a fresh round of crisis talks between Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov on the margins of the G20 summit in Hangzhou had ended without agreement.

A deal to provide aid to Aleppo's ravaged civilians and at least partially halt Russian and Syrian bombardments had looked likely on Sunday, before talks collapsed.

US officials accused Russia of backtracking on already agreed issues which Washington refused to revisit.

As so often in the five year conflict -- which has killed around 300,000 people and forced millions to flee -- the talks seemed overtaken by developments on the ground.

Syrian government troops encircled rebel-held parts of Aleppo, renewing a brutal siege.

Once Syria's economic powerhouse, Aleppo has been ravaged by the war that began with protests against President Bashar Assad's government in March 2011.

Presidents Barack Obama and Vladimir Putin were expected to meet later Monday, but it was far from clear that they could find agreement.

The White House has been reluctant to tether Obama to a deal that could well fail.

Earlier truces in Syria have rapidly deteriorated, and Obama warned Sunday that the US was approaching the talks "with some skepticism".

"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."

The White House is also reluctant to offer Putin a high-profile stage to gain international legitimacy after his backing for a regime that has used chemical weapons.

US officials had hoped to build pressure on Moscow over its support for Assad's government during the G20 and upcoming UN General Assembly.

They have gone as far as suggesting Moscow is complicit in war crimes.

Without Moscow and Tehran's support most experts believe Assad would have been unable to remain in power.

"You have the Assad regime which has been killing its own citizens with impunity, supported by the Russians and the Iranians," Obama said on Sunday.