This undated picture shows the Sirius Star conducting a trial run in South Korea. Somali pirates have hijacked the Saudi-owned tanker off the Kenyan coast, the U.S. navy said Monday. (Daewoo/Associated Press)

Somali pirates are in command of a Saudi supertanker in the Indian Ocean after staging their most daring hijacking yet off the coast of Kenya.

The Sirius Star, owned by the Saudi company Aramco, is the largest ship ever taken by the pirates, and can carry a cargo of crude oil worth more than $100 million.

Pirates boarded the Sirius Star on Saturday in the Arabian Sea, roughly 700 kilometres off the coast of Kenya.

The pirates struck hundreds of kilometres from their usual hunting grounds, and experts warn there aren't enough warships in the region to stop them.

The ship is the size of an aircraft carrier, and can hold more than two million barrels of crude oil.

The pirates are getting bolder, said Roger Middleton, who has studied piracy off the African coast for Chatham House, a London-based think-tank.

"This is much, much further south than Somali pirates have been active in the past. It's much further out into the ocean than they've been active in the past. So, it means that they're extending the range at which they're a threat."

Lt. Nathan Christensen, a spokesman for the U.S. Navy's 5th Fleet, said the Sirius Star was carrying crude at the time of the hijacking, but he did know how much. He also had no details about where the ship was sailing from and where it was headed at the time of the attack.

Christensen said the hijackers were taking the ship to an anchorage off Eyl, a northeastern Somali port town that is a haven for pirates and the ships they have seized.

The ship was sailing under a Liberian flag, and its 25-member crew includes citizens of Croatia, Britain, the Philippines, Poland and Saudi Arabia, Christensen said.

Piracy lucrative

Even before this latest seizure, shipping companies had become increasingly wary of Somali pirates.

A vessel from the Norwegian firm Odjfell SE narrowly avoided capture recently. The crew fought off pirates with the ship's water cannons.

Monday, the company announced it won't take any more chances. Its fleet of nearly 100 ships will avoid the Somali coast, and sail south around the Cape of Good Hope.

"We will no longer expose our crews to the risk of being hijacked in the area," said the company's president, Terje Storing.

Storing said more military ships are needed off Somalia to protect merchant shipping.

The European Union and the U.S. navy have warships in the area, but, with hijackings on the rise, the presence appears insufficient.

Somali pirates are trained fighters, often dressed in military fatigues, using speedboats equipped with satellite phones and GPS equipment. They are typically armed with automatic weapons, anti-tank rocket launchers and various types of grenades.

As pirates have become better armed and equipped, they have sailed farther out to sea in search of bigger targets, including the oil tankers, among the 20,000 tankers, freighters and merchant vessels passing through the Gulf of Aden each year.

Attacks have increased by more than 75 per cent this year.

With many attacks ending with million-dollar payouts, piracy is considered the most lucrative work in Somalia. Pirates rarely hurt their hostages, instead holding out for a huge payday.

The strategy has been effective: a report last month by a London-based think-tank said pirates have raked in up to $30 million in ransoms this year alone.

With files from the Associated Press