The United States and Russia on Sunday tried to finalise a ceasefire deal for Syria, a second this year after a first collapsed, but US president Barack Obama said "grave differences" remained and fighting on the ground continued.
A truce brokered by the two countries in February broke down and peace talks between the Syrian government and opposition ended earlier this year with both sides trading the blame.
Fighting in the five-year conflict has since escalated in many areas, especially around the embattled city of Aleppo in the north.
In a move separate to the fight between the Syrian government and its opponents, Turkey recently launched its first full-scale incursion into Syria since the civil war began, supporting rebels fighting Islamic State and Kurdish forces.
Speaking on the sidelines of the G20 summit in the Chinese city of Hangzhou, Mr Obama said the ceasefire deal, which concerns the conflict between Syrian president Bashar al-Assad and various rebels backed by regional and Western allies, still faced obstacles.
"We're not there yet," he told reporters after a meeting with British prime minister Theresa May.
“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,” Mr Obama said.
Russia and the United States have backed opposing sides in the conflict. Moscow has backed Dr Assad and Russian warplanes have been targeting the opposition for nearly a year, while Washington has supported some rebel groups fighting under the banner of the Free Syrian Army.
Russia’s deputy foreign Minister Sergei Ryabkov, also speaking on the sidelines of the G20, said 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.”
Ceasefire terms
A letter from Washington’s Syria envoy Michael Ratney to the Syrian armed opposition, a copy of which was seen by Reuters, told rebels an agreement could be announced soon, and laid out some of the terms.
The deal would oblige Russia to prevent Syrian government warplanes from bombing areas held by the mainstream opposition, and would require the withdrawal of Damascus’s forces from a key supply route north of Aleppo, the letter dated September 3rd said.
In return, the United States would coordinate with Russia in fighting against al-Qaeda, it said, without elaborating.
The letter stressed that for the deal to take effect, rebels would have to cooperate while guarantees were being sought that Moscow, Syria’s most powerful ally, would respect it.
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 two million people in government- and rebel-held areas.
Government forces would withdraw from a key supply route which previously led into the rebel-held east of the city and which was cut in July. The Castello Road would become a demilitarised zone, Mr Ratney’s letter said. Another route in southwest Aleppo would subsequently be used for aid delivery.
The government and rebels would also be required not to block any aid entering the city, it said.
The agreement 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.
The February ceasefire deal excluded Nusra Front and Islamic State. But Nusra Front severed ties with al-Qaeda in July. Earlier this year the opposition accused Moscow and Damascus of using the close proximity of Nusra Front to more moderate rebel groups as a pretext to bomb the latter.
Battles in north
Fighting in Syria has not slowed. Syrian government warplanes heavily bombarded areas of southwestern Aleppo recently captured by rebels on Saturday and Sunday, and there were ground battles as government forces and allies attempted to capture rebel-held territory, the Syrian Observatory for Human Rights reported.
Rebel official Zakaria Malahifji of the Fastaqim group, which operates in Aleppo, said battles raged in the area on Sunday.
To the northeast of Aleppo, Turkey-backed rebels pushed Islamic State out of areas around the town of al-Rai near the Turkish border, Mr Malahifji said, on the second day of an offensive launched from al-Rai.
Ten days Turkey launched its first full-scale incursion into Syrian soil since the conflict began, aimed at Islamic State and at US-backed Kurdish forces in the area, which have also been battling the jihadists.
Ankara considers the Kurdish YPG militia a terrorist group and is worried that YPG advances will embolden Kurdish militants on its own soil.