Intense air raids shook Syria's Aleppo on Wednesday as increasing tensions between Moscow and Washington poisoned efforts to revive a failed ceasefire.
The UN Security Council was due to meet later for talks on salvaging the truce -- which collapsed on Monday after only a week -- but the mood was likely to be tense after the United States said it held Russia responsible for a deadly air strike on an aid convoy.
US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov are both to address the council, ahead of more talks expected later this week.
Russia and the United States co-sponsored the ceasefire plan, with Kerry warning it could be the "last chance" to try to end Syria's civil war, which has killed more than 300,000 people in five years.
In Aleppo, a key battleground in the conflict, bombardment pummelled opposition-held districts and the city's outskirts into Wednesday morning.
The Syrian Observatory for Human Rights monitoring group said dozens of raids hit the city's east overnight, as regime troops advanced on rebels in Aleppo's southwestern outskirts.
It was a sleepless night for many Aleppo residents, AFP's correspondent in the city said, with the bombardment continuing until rain broke out over the city at dawn.
In the rebel-held neighbourhood of Sukkari, Abu Ahmad cleared rubble and shattered glass from his doorstep after bombardment demolished the six-storey building next door, killing his neighbours.
He had tea with the two brothers who lived in the building late the previous night.
"Just an hour after I left, a missile destroyed their whole building and they both died under the rubble," Abu Ahmad said.
Syrian state media reported that the city's government-held west had come under rebel shelling, which killed two people.
A medical aid group said four of its doctors were killed and a nurse critically wounded when an air strike hit a clinic in a village near Aleppo late Tuesday.
The Union of Syrian Medical Relief Organisations (UOSSM) said the clinic in the village of Khan Tuman was completely levelled in the 11:00 pm (2000 GMT) strike and more dead were feared to be buried under the rubble, the group added.
The Britain-based Observatory said nine medical staff affiliated with the Army of Conquest rebel alliance were also killed while working in the clinic.
According to the World Heath Organization, Syria is the most dangerous country in the world for health professionals, with 135 strikes on clinics and hospitals last year.
Monday's strike on the aid convoy near Aleppo killed 20 civilians and destroyed 18 trucks, the Red Cross said, and prompted the UN to suspend all humanitarian convoys across Syria.
"There only could have been two entities responsible, either the Syrian regime or the Russian government," President Barack Obama's national security spokesman Ben Rhodes said.
"In any event, we hold the Russian government responsible for air strikes in this space."
Two Russian SU-24 ground attack jets were operating in the area where the aid convoy was struck, another US official told AFP.
"The best evaluation we have is that the Russians carried out the strike," he added, speaking on condition of anonymity.The Russian foreign ministry said the "unsubstantiated, hasty accusations" seemed designed to "distract attention from the strange 'error' of coalition pilots."
This was a reference to Saturday's bombardment and killing of dozens of Syrian troops by the US-led coalition fighting the Islamic State group, an attack which Washington said was a mistake.
Despite the tensions, Kerry insisted that efforts to salvage the truce were "not dead", after a short meeting of the 23-nation International Syria Support Group (ISSG) in New York, where world leaders have gathered for the UN General Assembly.
Kerry's spokesman John Kirby said it had been agreed that "despite continued violence" diplomats would use the agreement between the United States and Russia as a basis for more talks.
The deal foresaw an end to fighting between President Bashar al-Assad's forces and non-jihadist rebels, aid deliveries to besieged areas and, if the ceasefire held for seven days, cooperation between Moscow and Washington in battling IS and other extremist groups.