Syrian government troops seized a military academy south of Aleppo city today, once again encircling the rebel-held districts in the east and placing them under siege, a monitor said.
The Syrian Observatory for Human Rights, a Britain-based monitor, said government forces backed by Syrian and Russian air strikes had now severed the alternate route into the rebel east that opposition forces opened up in August.
"The army took control of the artillery academy, so they control all the academies, and the eastern neighbourhoods are under a full siege," said Observatory director Rami Abdel Rahman.
There was no immediate confirmation in state media.
Government forces had been advancing in southern Aleppo for days, with the fighting already creating shortages for the estimated 250,000 people living in the rebel-held parts of the city.
But the capture of the academies, which straddle the road running to the eastern neighbourhoods, reinstates the government's encirclement of Aleppo.
Once Syria's economic powerhouse, Aleppo has been ravaged by the war that began with protests against President Bashar al-Assad's government in March 2011.
It has been roughly divided between rebel control in the east and government control in the west since mid-2012, and in recent months regime troops have gradually surrounded the city.
Elsewhere in the country, Turkish forces and Syrian rebels expelled the so-called Islamic State group from the last areas of the Syrian-Turkish border under their control, the Observatory said.
"IS has lost its contact with the outside world after losing the remaining border villages between the Sajur River... and Al-Rai," the Britain-based monitor said.
US, Russia struggling to reach deal on Syrian ceasefire
Meanwhile, US President Barack Obama said today that the United States and Russia were struggling to reach a ceasefire agreement on Syria as the two sides planned to meet again tomorrow.
"We're not there yet," Mr 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.
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 Mr Kerry emerged alone to say a couple of issues still needed to be resolved and the two sides would resume talks tomorrow. He did not elaborate.
Officials from the US and Russia, which back opposite sides in Syria's five-year-old civil war, have been meeting since Mr 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 so-called Islamic State and other militant groups.
The plan would need Russia to convince Mr 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 contributing to a rise in militant Islamist groups.
Moscow has backed Mr 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.
