A fire in a Paris apartment building early on Tuesday killed seven people and sent residents fleeing to the roof or climbing out their windows to escape, authorities said.

The cause of the blaze that burned for hours and injured at least 31 people is unclear.

Firefighters were still searching for other victims, city fire service spokesperson Clement Cognon told The Associated Press. He said the fire was extinguished before dawn but firefighters are still going door to door to ensure there are no more victims and prevent residual fires. Emergency workers are also seeking to shore up the damaged building to prevent collapse.

'The situation was already dramatic when the firefighters arrived,' Cognon said.

Firefighters rescued about two dozen people who had fled to the roof or climbed out windows to escape the flames, and evacuated others from inside the eight-storey building.

Flames shot out of the upper floors of the building, filling the night sky with smoke, in images of the operation released by the fire service.

About 50 people were safely rescued, he said. Among the injured were six firefighters.

'I heard a woman screaming in the street, crying and screaming for help,' said witness Jacqueline Ravier, who lives across the street. She saw a young man blackened by smoke and a woman motionless on the ground. She said flames were shooting out for hours from the top of the building and smoke-covered victims were fleeing.

She said shaken residents were brought to her building and the one next door while firefighters continued to fight the flames. 'We feel the smoke,' she said. 'What's surprising is how long it lasted.'

The building is on Rue Erlanger in the 16th arrondissement, one of the most high-end and calmest districts of Paris. It is close to the popular Bois de Boulogne park and about 1km from the Roland Garros stadium that hosts the French Open tennis tournament and near the Parc de Princes stadium that's home to Paris Saint-Germain, the country's top soccer team.

An investigation is under way, and the Paris prosecutor went to the scene.

More than 200 firefighters and emergency workers were involved in the operation, Paris police said, and the street was blocked off. Neighboring buildings were also evacuated.

The fire comes a month after a deadly explosion and blaze linked to a gas leak in a Paris bakery.