Preparations were under way to salvage a cargo ship packed with electric vehicles that caught fire off the Dutch coast, officials said Friday, in an operation intended to avert an ecological disaster.

An electric car is suspected of having sparked the deadly blaze and officials said earlier that nearly 500 electric vehicles were aboard, far more than initially reported.

Fire broke out on the Fremantle Highway late Tuesday, killing one member of the all-Indian crew and prompting a massive effort to douse the flames.

"The temperature on board the ship has dropped sharply and the intensity of the fire and smoke development have decreased," said the Rijkswaterstaat, the national water management agency, in a statement at about 11:30 pm (2130 GMT).

"The cargo ship is stable at this time. The ship is also still intact below the waterline and does not tilt."

The agency said it, as well as salvage companies, "have now started preparations for towing the freighter to an area further east", after rescuers were able to board the ship and connect it to a tug.

It added that towing the ship was likely to take around 12 hours, pulling the stricken vessel to a temporary anchorage north of Schiermonnikoog island -- "a better starting position for Rijkswaterstaat, the Coast Guard and the collaborating salvage companies".

The agency added that "no direct consequences" were expected for the surrounding environment, and the Fremantle Highway would eventually be towed to a port, which was yet to be determined.

While the timing would be affected by the weather and the state of the smoke, it was "likely that towing will begin this weekend".

Efforts to extinguish the blaze were halted Thursday to prevent the ship from losing stability due to the volume of water accumulating on board.

Japan-based K Line, the ship's charter company, reported there were 3,783 cars on board the vessel -- far more than an initial estimate of around 3,000.

These were "all brand new/no used cars on board" including 498 "electrical vehicle units", the company told AFP in a statement.

Ship owner Shoei Kisen Kaisha has said there was a "good chance that the fire started with electric cars", but added that the cause still needed to be investigated.

One sailor died after he and 22 others were rescued from the burning ship that had forced some crew members to jump overboard.

The blaze has raised the specter of an ecological disaster on a nearby chain of islands, which include Terschelling and Ameland, where the fire was first reported.

The ship remained close to Terschelling and Ameland, which are part of an archipelago of ecologically sensitive islands in the Wadden Sea.

The area spanning the Netherlands, Germany and Denmark has been declared a UNESCO World Heritage Site and has a rich diversity of more than 10,000 aquatic and terrestrial species.