Air strikes have hit rebel-held parts of Aleppo just hours into an announced 48-hour ceasefire.
Aleppo, Syria's largest city before the civil war with a population of more than two million people, has been divided for years into rebel and government sectors.
Capturing the city is one of President Bashar al-Assad's key strategic objectives.
Russia, an ally of Syria, announced the truce there on Thursday but did not say which parties had agreed to it.
There has been no public comment on the truce announcement from Assad's government or factions fighting his forces.
The Britain-based monitor the Syrian Observatory for Human Rights said aerial bombardments hit a number of neighbourhoods in the opposition-held sector and that there were reports of one death and some injuries.
"There were strikes on a number of residential areas causing fires and damage. The truce was supposed to have come into effect at 12 midnight but now there is no truce," Bebars Mishal, a civil defence chief working in rebel-held areas of Aleppo told Reuters.
Medical charity Medecins Sans Frontieres (MSF) said an air strike had put one of Aleppo's biggest hospitals out of service.
It was not immediately clear if strikes had hit the 64-bed MSF-supported Omar Bin Abdulaziz hospital directly or nearby, and the extent of damage was not known.
MSF condemned the continued attacks on healthcare in Syria on its Twitter account and said it was "extremely worried about (the) potential impact on people's access to healthcare" in the city.
Two residents in Aleppo said there had been a number of barrel bombs dropped from helicopters and aerial attacks on Thursday within the city.
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