Seven people, including three children and their mother, died early on Thursday in what authorities suspect was arson that devastated a crowded seventh-floor apartment in a building in Nice, the largest city on the French Riviera.

The fire started around 3 a.m. (0100 GMT) and spread so quickly through the apartment that only the people who were close to the windows could be saved, Hugues Moutouh, the local representative of the interior ministry told TV station BFM.

The fire killed one man, a teenager, two women and three children, he said. One of the victims jumped through the window tied to a mattress, he added.

"The family's mother went to take her children to safety beside the window and then went back into the blaze to get her three little ones and never came back," he told reporters.

The city's prosecutor Damien Martinelli opened an investigation into a possible voluntary and criminally motivated fire.

"I will have to give more elements on the investigation later but we are starting with an arson," he said.

The city mayor, Christian Estrosi, said people wearing balaclavas were spotted by CCTV footage in the area.

The building is located in the Moulins neighbourhood, an area of run-down social housing towers, similar to many "banlieues" in France.

Acting Prime Minister Gabriel Attal and acting Interior Minister Gerard Darmanin will travel to the area.