Top diplomats from the United States and Russia failed to reach a deal to ease fighting in Syria on Monday, 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 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.
— Department of State (@StateDept) September 4, 2016
This is the second time in two weeks that Kerry and Lavrov failed to reach a ceasefire deal. They last met in Geneva on August 26.
Kerry previously said the United States won't accept a deal just to have it collapse again.
A cessation of hostilities agreement brokered by Lavrov and Kerry in February unraveled within weeks, with Washington accusing Syrian regime leader Bashar al-Assad's forces of violating the agreement.
The next time they are likely to meet is on the sidelines of the UN General Assembly meetings in New York from September 18.
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.
On Sunday, Syrian regime 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 the Assad regime in March 2011.
Later on Monday, Presidents Barack Obama and Vladimir Putin met on the margins of the G20 summit in China, the White House said, hours after doplomats of their respective countries failed to reach a deal on Syria.
"The President is taking part in a pull-aside with President Putin of Russia," said National Security Council spokesman Ned Price. "We expect to provide additional details of their discussion later today."
Earlier truces in Syria have rapidly deteriorated, and Obama warned on 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."
The White House is also reluctant to offer Putin a high-profile stage to gain international legitimacy due to his backing of a regime that has used chemical weapons.
US officials had hoped to build pressure on Moscow over its support for Assad's regime 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.