An engine room fire on the Costa Allegra knocked out the ship's main power supply in the Indian Ocean on Monday, leaving it adrift with more than a thousand people on board in waters vulnerable to pirate attacks.

The ship's Italian owner, Costa Cruises, a unit of U.S. cruise line giant Carnival Corp, said a plan to tow it to the nearer island of Desroches had been aborted because it would have been harder to moor and disembark the passengers there.

The Trevignon, a deep sea trawler which sails the oceans for tuna from the Atlantic port of Concarneau, is pulling the Costa Allegra, a vessel many times its size, on a 400-metre cable at a speed of only about six knots, the Trevignon's skipper Alain Dervout told his local French newspaper, Ouest-France.

He was joined on Tuesday by two tugs and a coastguard ship, all from Seychelles, the archipelago's government said. A military aircraft was also flying in support of the operation.

The cruise ship was due to arrive at the Seychelles capital of Victoria on Wednesday evening or Thursday morning, depending on weather conditions, government spokeswoman Srdjana Janosevic said. Clocks in the Seychelles are four hours ahead of GMT.

"Helicopters will ensure continuous supply of food, comfort items, flashlights in order to mitigate guests' discomfort given the difficult conditions on board," Costa Cruises spokesman Davide Barbano said in a statement.

A team from the Italian coastguard is heading to the Seychelles to investigate the accident, but a spokesman for the agency it would be wrong to make analogies to the Costa Concordia disaster on January 13, in which at least 25 people died and over which a criminal investigation has been launched.

"They are two different situations, totally different conditions, so they are not related accidents," Cosimo Nicastro told Reuters.

Prosecutors in the Italian city of Genoa have opened an investigation into the fire on the Costa Allegra, judicial sources said.

Nicastro said there was no question of the passengers being transferred to other vessels.

"The safest place for the people is on the ship. There is no reason to put them on another ship or a helicopter. They will remain on the Costa Allegra and we will keep monitoring the situation," he said.

An evacuation off Desroches Island would have presented the ship owner and local authorities with a tricky and expensive logistical operation.

The 636 passengers and 413 crew would have had to use the ship's lifeboats to land on the exclusive coral-fringed island, where Britain's Prince William and his then girlfriend, now wife, Kate Middleton, stayed a few years ago.

"Logistics and hotels on the island are not sufficient. It would require ... an immediate transfer from Desroches to Mahe," Barbano, the Costa Cruises spokesman, said.

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Seychelles authorities still face a logistical headache finding accommodation in Mahe for all those onboard.

"Right now we are in consultation with the hotels on Mahe to find out how many beds are available. It's a busy time of year," government spokeswoman Janosevic told Reuters.

The giant Costa Concordia capsized after hitting rocks off the Italian island of Giglio, killing at least 25 people. Divers and rescue workers are searching for the bodies of seven still missing more than six weeks after the disaster.

The Costa Allegra, at 29,000 tonnes several times smaller than the Concordia, was sailing some 200 miles southwest of Seychelles when the fire broke out and it sent a distress signal, Costa Cruises said.

The fire was put out and there was light onboard thanks to an emergency battery. But there was no air conditioning to counter tropical temperatures, nor cooking facilities.

The passengers, including four children, are from 25 different nations, with the largest contingents being 127 from France and 126 from Italy. There are 38 Germans, 31 Britons, 13 Canadians and eight Americans on board.

A former British colony with historic ties to France, the islands of Seychelles are home to a little over 80,000 people. The Costa Allegra left Diego Suarez in Madagascar on Saturday and, sailing northeast, had been due to dock on Mahe on Tuesday.

It is being protected by nine members of an anti-piracy unit of the Italian navy, a precaution regularly taken on ships in the Indian Ocean which is prone to attacks by Somali pirates.

"The ship is not in a high-risk area, but we can't be 100 percent sure," said Costa Cruises' Giorgio Moretti.

While yachts have been seized in the past near Seychelles, pirates have yet to successfully hijack a cruise liner in the Indian Ocean.

Shares of Costa Cruises' parent company Carnival (CCL.N) were down 29 cents at $29.67 in morning trade in New York.

Costa was accused by some passengers of long delays and a lack of organisation in the evacuation of the Costa Concordia.

That vessel's Italian captain is under house arrest near Naples accused of multiple manslaughter and abandoning the ship.

(Additional reporting by Catherine Hornby, Gavin Jones, Barry Moody, Antonio Denti in Rome and Richard Lough in Nairobi; Writing by Richard Lough; Editing by David Clarke and Alastair Macdonald)

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