Nuclear reactor number 1 ('Flamanville 1') of the Flamanville nuclear plant after an explosion at the plant in Flamanville, northwestern France Charly Triballeau/AFP

An explosion hit EDF's Flamanville nuclear power plant in Flamanville, northern France, on Thursday, leaving several people injured.

Authorities said that five people had been treated for smoke inhalation, but there was no risk of nuclear contamination. "It is a significant technical event but it is not a nuclear accident," senior local official Olivier Marmion told AFP.

Firefighters were alerted to a fire and an explosion in the engine room around 10am local time and emergency services were sent to the location, according to Ouest France newspaper, citing local police.

Marmion, director of the local prefect's office, told the newspaper that the explosion occured outside the plant's "nuclear zone", and a special evacuation plan for the region in the event of a nuclear incident had not been activated.

EDF said that fire crews had brought the blaze under control by 11am local time.

France's flagship nuclear power plant is located in the Cotentin Peninsula, near the English Channel and houses two pressurised water reactors that produce 4% of France's electricity. One reactor was shut down after the explosion and remains off line.

The cause of the explosion remains unknown, but authorities have ruled out sabotage.

A third reactor at the facility scheduled to be completed in 2018 will be the world's largest.

Work on the third reactor began in 2007, but has yet to be completed with the project reportedly running three times over budget and France's nuclear safety authority recently finding weaknesses in the reactor's steel.