Hangzhou (China)/Beirut: US President Barack Obama said on Sunday that the United States and Russia were struggling to reach a ceasefire agreement on Syria as the two sides planned to meet again on Monday.
"We're not there yet," Obama told reporters after a meeting with British Prime Minister Theresa May on the sidelines of the G20 summit in the Chinese city of Hangzhou.
"We have grave differences with the Russians in terms of both the parties we support but also the process that is required to bring about peace in Syria," he said.
On the battlefront, fighting raged. Syrian government forces and their allies scored an important victory by recapturing areas in southwestern Aleppo which rebels had seized last month, the Syrian Observatory for Human Rights reported.
An agreement that would stop the fighting and allow more humanitarian deliveries looked set to be announced by US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov in Hangzhou.
Two lecterns had been set up in a room for a news conference. But Kerry emerged alone to say a couple of issues still needed to be resolved and the two sides would resume talks on Monday. He did not elaborate.
Officials from the United United States and Russia, which back opposite sides in Syria's five-year-old civil war, have been meeting since Kerry travelled to Moscow in July with a proposal that would halt the fighting.
It would ensure that government fighters pulled back in some areas, including around Aleppo, to allow convoys of humanitarian aid to reach civilians caught in the fighting.
The ceasefire would be overseen through Russian-US intelligence sharing and military cooperation that would focus on going after IS and other militant groups.
The plan would need Russia to convince President Bashar Al Assad to agree on grounding his air force, a move that Russian Foreign Minister Sergei Lavrov has said was not the goal.
The war has killed more than 250,000 people and displaced 11 million, causing a refugee crisis in the Middle East and Europe, and contributed to a rise in militant groups.
Moscow has backed Assad in the war and Russian warplanes have targeted the opposition for nearly a year, while Washington has supported some rebel groups fighting under the banner of the Free Syrian Army to topple him.
A truce brokered by the Cold War foes in February broke down and peace talks between the government and opposition ended in April with both sides trading the blame. Fighting has since escalated in many areas, especially around Aleppo in the north.
Kerry said he would not rush into any agreement just to see it fail again. A senior State Department official, who declined to be named, said Russia had walked back on some of issues that the sides had already agreed on, which is why both sides need to continue talking.
"If we do not get some buy-in from the Russians on reducing the violence and easing the humanitarian crisis, then it's difficult to see how we get to the next phase," Obama said.
The White House has said Obama and Russian President Vladimir Putin likely would have the chance to talk informally on the sidelines of the G20.
Kerry said it remained to be seen whether the sides could agree on a deal.
"There are a couple of tough issues that we talked about today that we will go back and review, I will go back and review, and we've agreed to meet tomorrow morning and see whether or not it is possible to bridge the gap, come to conclusion on those couple of issues," Kerry said.
Russia's Deputy Foreign Minister Sergei Ryabkov, also speaking in Hangzhou, said earlier that a deal was close but that the timing of any announcement could not be predicted.
"We are talking about most serious issues of implementing a ceasefire," he said. "We are close to the deal...but art of diplomacy requires time to implementation. I can't tell you when the agreement will be reached."
A letter from Washington's Syria envoy Michael Ratney to the Syrian armed opposition, a copy of which was seen by Reuters, laid out some of the ceasefire terms.
It would oblige Russia to prevent government warplanes from bombing areas held by the mainstream opposition, and would require the withdrawal of Damascus's forces from a supply route north of Aleppo, the letter dated September 3 said.
In return, the United States would coordinate with Russia in fighting against Al Qaeda, it said, without elaborating.
The deal would focus on delivery of humanitarian supplies to Aleppo, where recent advances by both sides have cut supplies, power and water to nearly 2 million people in government- and rebel-held areas.
It also required the Syrian government and Russia to avoid bombing opposition-held areas - including where more moderate insurgent groups are operating close to Jabhat Fatah Al Sham, previously the Al Qaeda-affiliated Nusra Front.
Government forces and their allies recaptured areas in southwestern Aleppo on Sunday from the rebels after heavy bombardments and repeated attempts to drive the insurgents back.
The British-based Observatory said the retaken areas included the Weaponry College and the Air Force Technical College in Aleppo's southwestern outskirts.
To the northeast of Aleppo, Turkish-backed rebels pushed IS out of areas around the town of Al Rai near the Turkish border on the second day of an offensive launched from Al Rai.
Turkey last month mounted its first full-scale incursion into Syrian territory since the conflict began, aimed at IS and at US-backed Kurdish forces in the area, which have also been battling the militants.
Ankara fears that advances by the Kurdish YPG militia will embolden Kurdish militants on its own soil.
Turkish Prime Minister Binali Yildirim said on Sunday that Turkey will never allow the formation of an "artificial state" in northern Syria.
"We will never allow the formation of an artificial state in the north of Syria," Yildirim said in a speech in the southeastern city of Diyarbakir, where he announced an investment programme to rebuild parts of the largely Kurdish region that have been destroyed by security operations.
"We are there with Euphrates Shield, we are there to protect our border, to provide for our citizens safety of life and property, and to ensure Syria's integrity."
Yildirim's comments echoed those of President Tayyip Erdogan at the G20 gathering of world leaders in China, who told reporters following a meeting with US President Barack Obama: "It is our wish that a terror corridor not be formed across our southern border".