The rebel-held eastern half of Syria's largest city was once again cut off by government advances on Sunday, a monitoring group said, as attempts by the US and Russia to close a deal on the Syrian conflict faltered.

The development means that an estimated 250,000 to 300,000 remaining residents in eastern Aleppo are again facing siege.

Before rebels broke through government lines to open a new route into the east a month ago, the UN had warned that food supplies there would only last until mid-August.

But government forces backed by Russian and Syrian air raids on Sunday captured that route, through the grounds of military academies in the city's southern Ramousseh district, the Syrian Observatory for Human Rights said.

The government's SANA news agency said that the military academies had been completely recaptured, cutting off all rebel transport and supply lines between eastern Aleppo and the rebel-held countryside south of the city.

Efforts by the UN to reach agreement on regular 48-hour ceasefires to allow aid deliveries to the city have so far been unsuccessful, despite government ally Russia agreeing to the plan.

The renewed siege of eastern Aleppo comes a week after a symbolic victory for the government's widely condemned siege tactics, when the rebel-held Damascus suburb of Daraya was evacuated after a four-year siege that, according to the UN, at one point reduced residents to eating grass.

Under pressure from the siege and government military advances, Daraya's rebel fighters were bussed to opposition-held northern Syria while the remaining residents were moved to shelters and President Bashar al-Assad's forces took over the empty town.

High-level diplomacy meanwhile stumbled as US Secretary of State John Kerry and Russian foreign minister Sergei Lavrov decided to meet again on Monday for further efforts to reach agreement on a Syria deal, despite earlier reports that an agreement was close.

Kerry told reporters, after talks with Lavrov on the sidelines of Group of 20 (G20) summit in Hangzhou, China, that there were still "a couple of tough issues" to work out and that efforts would continue to "see whether or not it is possible to bridge the gap."

"We're not going to rush and we’re not going to do something that we think has less than what we believe is a legitimate opportunity to be able to try to get the job done," he said.

CBS News earlier reported that a deal was near that would involve military and intelligence cooperation to fight terrorist groups in exchange for Russia pressuring al-Assad, a longtime Moscow ally, to stop attacking civilians and allow humanitarian aid.

President Barack Obama said Sunday the US was "not there yet" in talks with Russia about a cessation of hostilities in Syria, but that progress was possible as "around-the-clock" negotiations between US and Russian officials were underway.

Obama pointed to "grave differences" with Russia over its support of the Syrian regime and the path forward.

"We're not there yet. And understandably, given the previous failures of cessations of hostilities to hold, we approach it with some scepticism," he said. "But it is worth trying."

Syria's conflict started with peaceful anti-government protests in March 2011.

It has now spiralled into a four-way civil war between al-Assad's government, mainly Islamist rebels, the Islamic State extremist group, and leftist Kurdish forces.

Some 4.8 million of Syria's pre-war population of 22.4 million have fled abroad, with another 8.7 million predicted to be displaced inside the country in 2016, according to the UN refugee agency.

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