Seven people have died, including three young children and a teenager, in a fire that broke out in the early hours of Thursday in the southern French city of Nice.

The local prosecutor said officials were investigating the blaze as a potential arson attack. The mayor of Nice said they had images that clearly showed the fire had been caused by petrol poured into a stairwell by hooded people during the night.

Flames engulfed the upper floors of an apartment block in the Moulins area of Nice, close to the city’s airport.

Interior Minister Gérald Darmanin said three young children were among the victims and that police would “shed light on the circumstances of this terrible tragedy”.

The fire broke out on second floor and spread quickly to the upper floors. Nice prosecutor Damien Martinelli told journalists that in light of initial findings, “I have opened an inquiry into acts of arson as the cause of death.”

Local reports said the seven victims were all part of the same family. Officials said three adults, a teenager and younger children aged five, seven and 10 had died.

Two members of the family are said to have tried to escape through a window – one died and the other is in a critical condition.

Prefect Hugues Moutouh said everyone had been affected by the tragedy and “very determined to provide help and assistance to the survivors”.

One man called Iftahou told the Nice Matin website that he had arrived at the scene while the fire was burning and realised there was no hope for those inside.

The head of the Comorian community in the south of France, Najim Maecha, said he knew the family and that they had lived in Nice since about 2013.

Nice Mayor Christian Estrosi said the human toll of the disaster was extremely high. As well as the seven victims, another 30 people have suffered smoke inhalation.

“I’ve asked the interior minister to deploy security forces to ensure residents’ safety and avoid new incidents after this tragedy that’s being investigated as potential arson,” he said.

President Emmanuel Macron, on a visit to the UK, said “our thoughts are with the victims’ relatives. We are on their side and on the side of the people of Nice”.