Updated 3.50pm

Three rescuers from the French lifeboat service died Friday when their vessel capsized in rough seas caused by a giant storm pummelling the country's Atlantic.

"In late morning, a boat from the SNSM capsized 800 metres from the coast around the Sables d'Olonne with seven people on board," the local prosecutor's offices said in a statement sent to AFP.

Three of them died while four managed to swim to shore, the statement added.

Three helicopters and around 60 firefighters were attempting to find the bodies of the rescuers near the site of the accident about 60 kilometres north of the port of La Rochelle.

The team from the National Society of Sea Rescue (SNSM) had been attempting to help a fishing boat which was struggling in the giant swell caused by storm Miguel, which is packing winds of up to 120 kilometres an hour.

Photos from the scene show waves crashing over the front of the small rescue boat as it headed out.

After capsizing, it ended up upside down on the rocky shore.

The captain of the fishing boat is also missing.

Storm Miguel has led the French weather service to put large areas of the Atlantic coast on alert for high winds and heavy rain.

The unseasonal storm hit the north-west Spanish region of Asturias late on Thursday, and swirled around the Bay of Biscay.

It is due to move north and hit Wales later on over the weekend.