An apartment building blaze killed seven people in the southern French city of Nice and police were investigating the fire as a possible arson, authorities said.

The dead include three children - five, seven and ten years old - and a 17-year-old teenager who tried to escape by jumping from a window, they said.

The apartment was occupied by a family believed to have Comoran origins, the regional prefect Hugues Moutouh said, referring to the southern African island nation.

A man reacts outside the building as firefighters work to extinguish the blaze

Ten people were inside when the fire broke out.

Rescuers were alerted at around 2.30am (1.30am Irish time) to the blaze on the seventh floor of the building in the working-class neighbourhood of Les Moulins in the west of the city.

Despite the substantial resources deployed, "unfortunately seven people died during this fire", firefighters said.

Interior Minister Gerald Darmanin said on social media that the quick arrival of firefighters "probably prevented more deaths".

Nice prosecutor Damien Martinelli said investigators were looking into a "criminal" cause for the fire.

"In light of the initial evidence, I have opened an investigation into acts of arson leading to death," he told reporters at the scene.

Rescuers said they were confronted by a 'violent apartment fire' on the seventh floor

The blaze probably broke out on the building's second floor and spread to higher floors, authorities said.

Rescuers said that three people were taken to hospital, one of them with life-threatening injuries.

They said firefighters were confronted by a "raging apartment fire" on the seventh floor of the building. They carried out three aerial ladder rescues and evacuated dozens.

In total, 25 fire engines and 72 firefighters were involved in tackling the fire.

Another 20 people were evacuated to a temporary shelter, with Nice Mayor Christian Estrosi saying a crisis unit to help anyone affected by the fire.