An intense bombing campaign has targeted several areas in the rebel-held part of Aleppo city, just hours after the US and Russia ended any pretence of their ceasefire for Syria remaining in force.
The air campaign overnight came after Syria's military command announced it was launching operations in Aleppo's rebel-held eastern quarters, raising concerns of imminent ground operations.
Rami Abdurrahman, of the Britain-based Syrian Observatory for Human Rights, said that government troops seized buildings on the frontline, pushing back rebel fighters in the southern al-Amiriah district.
Ibrahim Alhaj, a member of the Syrian Civil Defence, confirmed the government troop movements.
He said the bombing targeted two civil defence centres, putting one out of service.
After days of increasing violence in Syria, US secretary of state John Kerry and Russian foreign minister Sergey Lavrov were set to hold more talks on Friday in a bid to try to resuscitate the ceasefire.
But after three days of private and public diplomacy on the sidelines of the United Nations General Assembly, Mr Kerry bluntly told reporters: "We can't go out to the world and say we have an agreement when we don't."
As the diplomats huddled in a New York hotel, Syria's military command announced it would restart operations in Aleppo.
According to one official present in the gathering, Mr Kerry was informed of the news when his chief of staff showed him a headline on his BlackBerry.
A furious Mr Kerry then told the entire room, Mr Lavrov included, that "even while we are meeting here, they are doing this", said the official.
Mr Lavrov told Russian media that consultations would continue to "guarantee" the ceasefire.
But even as Mr Kerry vowed to press on with all efforts to find a peaceful solution to the war between Syrian president Bashar Assad's Russian-backed government and US-backed rebels, he acknowledged the current strategy was not working.
"We can't be the only ones trying to hold this door open," Mr Kerry said. "Russia and the regime must do their part or this will have no chance."
He called for the immediate grounding of planes and helicopters that have launched air strikes, including a Russian one earlier this week that the US says hit an aid convoy, killing 20 civilians. Russia has denied responsibility, while raising a range of ulterior scenarios for how the caravan might have been struck.
"Absent a major gesture like this, we don't believe there is a point to making more promises or issuing more plans or announcing something that can't be reached," Mr Kerry said, describing a "moment of truth" for Syria, Russia and all those trying to halt the bloodshed.
Mr Lavrov had sought a three-day pause in fighting to revive the ceasefire but US officials said there was no point returning to a situation in which rebels would be pressed to hold fire, while the Syrian and Russian military could violate the agreement.