Three rescuers from the French ocean rescue service have died when their vessel capsized in rough seas as a giant storm pummelled the country's Atlantic coast.

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

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

"These tragic deaths have sparked deep emotions within the ocean rescue services," said SNSM chief, Xavier de la Gorce.

Three helicopters and around 70 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.

Maritime officials said the crew was using one of the SNSM's "all weather" vessels, specially designed to be unsinkable and able to automatically right itself if capsized.

Photos taken before the accident show waves crashing over the front of the small 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, and officials said he probably had crew members on board as well.

Rescuers and maritime affairs officer remove parts of a boat from the shore of Les Sables-d'Olonne

"So far we haven't been able to find the fishing boat. We've found fragments of the boat and an empty lifeboat," the regional maritime authority told AFP.

Yannick Moreau, the mayor of Les Sables d'Olonne, said the missing fisherman, who was retired, used to go out and catch shrimps to supplement his pension.

Authorities said the rough sea conditions were at level five of the nine on the Douglas scale, indicating waves of up to four metres high.

Mr Moreau said it was "incomprehensible that any boat went out on a day like today."

The SNSM gets funding from the state but is a non-profit organisation run by around 8,000 volunteers.

"The SNSM boat at Les Sables is a crew of professionals, sailors who know the ocean, know the conditions," Mr Moreau said, adding that his town was "in shock."

A number of government officials, including Interior Minister Christophe Castaner, sent their condolences and voiced their solidarity with the SNSM.

"My thoughts are with the mourning rescuers whose comrades were trying to help a boat in difficulty," he wrote on Twitter. 

Transport minister Elisabeth Borne, speaking in front of the National Assembly, described the incident as "a terrible drama".

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