A 75-year-old Frenchman attempting to row across the Atlantic, has not given any sign of life since early Friday when he was near the Portuguese archipelago of Madeira, his friends said Saturday.

Former paratrooper and avid triathlete Jean-Jacques Savin set off from mainland Portugal's southern tip on January 1, hoping to reach the Caribbean.

It was just his latest adventure after crossing the Atlantic alone in a custom-built barrel in 2019, a 127-day trip followed by thousands on Facebook.

"We're very worried," a team of France-based volunteers following his odyssey told AFP in a statement.

French adventurer Jean-Jacques Savin, 75, has not been heard of since early Friday French adventurer Jean-Jacques Savin, 75, has not been heard of since early Friday Photo: AFP / Philippe Lopez

"We haven't heard from him since 00:34 yesterday (Friday) morning," when he released "two distress beacons, telling us he was 'in great difficulty'."

His daughter in a Facebook post said a search operation "was immediately set in motion in coordination with the French, Portuguese and US sea rescue services".

He was last heard of north of the Madeira islands off the northwest coast of Africa, and heading towards Ponta Delgada in the Portuguese archipelago of the Azores.

Shortly after leaving on January 1, unfavourable wind conditions forced the adventurer to extend his trip by 900 kilometres (550 miles).

The former paratrooper set off from the southern tip of mainland Portugal on January 1 The former paratrooper set off from the southern tip of mainland Portugal on January 1 Photo: AFP / Philippe Lopez

On Wednesday, he had reported "strong swell and... wind" on Facebook, adding he had been forced to switch from using an electric water desalinator to a backup one operated by hand.

"It's costing me physical energy," he wrote. But "be assured, I am not in danger".

Savin, who said rowing across the Atlantic was a way to "laugh at old age", celebrated his 75th birthday on Friday last week on board his two-cabin canoe.

The vessel, which is eight metres long and 1.70 metres wide, has a rowing station at its centre.

He left with it packed with 300 kilos (660 pounds) of equipment, including freeze-dried food, a spear gun to fish and a heater, as well as the two desalinators.

To celebrate his birthday, he had also brought along his mandolin, foie gras and champagne.

"I am off on holiday to the open sea," he had said before leaving.

"I will row eight hours a day and when the wind blows too hard, I'll close myself in," he said.

He was looking forward to an extra safety measure that he did not have during his last barrel crossing: a tracker to make him visible to the radars of passing cargo ships.