LISBON (REUTERS) - Firefighters are struggling to put out a fire that broke out on Wednesday (Feb 16) on a vessel carrying thousands of luxury cars, which is adrift off the coast of Portugal's Azores islands, a port official said, adding it was unclear when they would succeed.

The Felicity Ace ship, carrying around 4,000 vehicles including Porsches, Audis and Bentleys, some electric with lithium-ion batteries, caught fire in the middle of the Atlantic Ocean on Wednesday.

The 22 crew members on board were evacuated on the same day.

"The intervention (to put out the blaze) has to be done very slowly," Mr João Mendes Cabeças, captain of the nearest port in the Azorean island of Faial, told Reuters late on Saturday. "It will take a while."

Lithium-ion batteries in the electric vehicles on board are "keeping the fire alive", Capt Cabeças said, adding that specialist equipment to extinguish it was on the way.

It was not clear whether the batteries sparked the fire.

Volkswagen, which owns the brands, did not confirm the total number of cars on board and said on Friday it was awaiting further information.

Ship manager Mitsui OSK Lines Ltd did not immediately respond to a request for comment.

Capt Cabeças previously said that "everything was on fire about five metres above the water line" and the blaze was still far from the ship's fuel tanks.

It is getting closer, he said.

"The fire spread further down," he said, explaining that teams could only tackle the fire from outside by cooling down the ship's structure as it was too dangerous to go on board.

They also cannot use water because adding weight to the ship could make it more unstable, and traditional water extinguishers do not stop lithium-ion batteries from burning, Capt Cabeças said.

The Panama-flagged ship will be towed to a country in Europe or to the Bahamas, but it is unclear when that will happen.