Obama says “grave differences” remain, despite earlier reports that the two sides were close to a truce deal.
The rebel-held half of Aleppo in northern Syria has once again been cut off by government forces, after heavy bombardments and repeated attempts to drive rebel fighters out of territory they took last month.
Pro-government forces, backed by heavy Russian air strikes, recaptured on Sunday a strategic military complex in the Ramosa district on Aleppo’s southwestern outskirts, according to Syrian state media and a monitor.
Rebel forces had captured the complex last month in an assault that broke through a government siege on Aleppo’s opposition-held eastern neighbourhoods – home to some 300,000 people.
The government takeover of the area has effectively reimposed the siege on the city’s rebel-held neighbourhoods.
“The army has managed now to control many areas within the vast complex that houses military installations. Rebel factions say that they are going to reunite in order to launch a counter offensive,” Al Jazeera’s Hashem Ahelbarra, reporting from the city of Elbeyli on the Turkish side of the Syria-Turkey border, said.
“Ramosa is crucial for all parties. It is crucial for the government because it will pave the way for them to encircle rebel-held areas. It’s also crucial for the rebels because it is the only supply line.”
Once Syria’s largest city, Aleppo has been roughly divided between government control in the west and rebel control in the east since mid-2012.
Syrian state television, citing a military source, said on Sunday that “armed forces, in cooperation with their allies, took full control of the military academy zone south of Aleppo and are clearing the remaining terrorists from the area”.
The advance, it said, had “cut all the supply and movement routes for terrorist groups from southern Aleppo province to the eastern neighbourhoods and Ramosa”.
Zakaria Malahifji, of the Fastaqim rebel group, confirmed to Reuters news agency that the Weaponry College and the Air Force Technical College in Ramosa had been recaptured by government forces.
And the UK-based Syrian Observatory for Human Rights, which monitors the war, said later on Sunday that government forces had also recaptured the Artillery College, and that the eastern part of the city had been cut off.
Russia’s intervention last year turned the war in President Bashar al-Assad’s favour in many areas, but rebels have made some gains lately in Aleppo and in Hama province further south.
A spokesman for rebel group Jaish al-Nasr, Mohammed Rasheed, told Reuters that rebels had launched a push to try and capture the town of Maan, north of the province capital, Hama city.
Advances by the rebels in recent days have brought them to within only six miles of government-controlled Hama.