French authorities say their troops have intercepted a ship thought to be carrying tons of cocaine off Africa's Atlantic coast.

Sources involved in the raid said the ship's crew may have thrown at least one ton of the drug overboard when they realised the raid was happening, although naval officers were able to recover around 100 kilogram from the sea.

The French ships fired warning shots in the air before aiming cannons at the freighter, French maritime officials said.

The ship, called the Winner, was registered in Cambodia and could have been carrying as much as two tonnes of cocaine, with a street value of 243 million euros ( $230m), according to the French news agency AFP.

Shots

"It was necessary to fire warning shots and then fire at the ship in order to seize control of it," the French prime minister's office said.

The Winner was allegedly carrying tonnes of cocaine

One member of the cargo ship's 12-member crew is reportedly injured and being treated by a navy doctor.

French maritime authorities said they had been monitoring the ship for several days, in a joint operation with the United States, Greece and Spain.

Navy vessels are escorting it to the French port of Brest, because the authorities said they could not conduct a thorough search at sea.