Alwahy News - Pirates seize Saudi oil tanker

The 318,000-tonne vessel, launched earlier this year, is flagged in Liberia and operated by Vela International.

"This represents a fundamental change in the ability of the pirates to be able to attack large vessels off the coast," Christensen told Al Jazeera.

"This attack is obviously so different because of the size of the the vessel - three times the size of a US aircraft carrier ... I dont have specifics as to what is happening now. The vessel itself is going to remain under pirates control."

The ship is carrying 25 crew members from Croatia, Britain, the Philippines, Poland and Saudi Arabia, the statement said.

Christensen said the fate of the crew was unknown.

He said the latest hijacking occurred despite a 25 per cent drop in attacks by pirates since August.

The International Maritime Bureau has reported that at least 83 ships have been attacked off Somalia since January, of which 33 were hijacked.

Of those, 12 vessels and more than 200 crew were still in the hands of pirates.

European operation

Last week, the European Union started a security operation off the coast of Somalia, north of Kenya, to combat growing piracy and protect ships carrying aid agency deliveries.

It is the EU's first-ever naval mission.

Dubbed Operation Atlanta, the mission, endorsed by the bloc's defence ministers at talks in Brussels, is being led by Britain, with its headquarters in Northwood, near London.

Pirates are well organised in the area where Somalia's northeastern tip juts into the Indian Ocean, preying on a key maritime route leading to the Suez Canal through which an estimated 30 per cent of the world's oil transits.

They operate high-powered speedboats and are heavily armed, sometimes holding ships for weeks until they are released for large ransoms paid by governments or owners.

Nato warships, along with ships and aircraft from several other nations, have been deployed in the region to protect commercial shipping.

Somalia has had no effective government since the 1991 overthrow of Mohamed Siad Barre, the former president, touched off a bloody power struggle that has defied numerous attempts to restore stability.