DUBAI, United Arab Emirates — Somali pirates hijacked a supertanker off the Kenyan coast, seizing the Saudi-owned ship loaded with crude and its 25-member crew, the U.S. Navy said today.

Lt. Nathan Christensen, a spokesman for the U.S. Navy's 5th Fleet, said the pirates hijacked the Sirius Star — a newly commissioned ship owned by Saudi oil company Aramco — more than 450 nautical miles southeast of Mombasa, Kenya.

Somali pirates have seized several ships off the Horn of Africa coast in the past week, but the latest hijacking — of a tanker the size of an aircraft carrier — marked a dramatic escalation.

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 rockets launchers and various types of grenades.

The tanker seized Saturday is 1,080 feet and can carry about 2 million barrels of oil.

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

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

A British Foreign Office spokesman said there were at least two British nationals aboard the MV Sirius Star, but said he could offer no further details on the ship or what had happened to it.

The Sirius Star was built in South Korea's Daewoo shipping yards and commissioned in March. Classed as a Very Large Crude Carrier, the ship is 318,000 dead weight tons.

An operator with Aramco said there was no one available at the company to comment after business hours.

Calls went unanswered at Vela International, the Dubai-based marine company that operated the ship for Aramco.

As pirates have become better armed and equipped, they have sailed farther out to sea in search of bigger targets, including oil tankers.

A NATO flotilla of seven ships, as well as a Russian frigate and Indian vessels, are in the Gulf of Aden to help the U.S. 5th Fleet in anti-piracy patrols and to escort cargo vessels. The 5th Fleet said it has repelled about two dozen pirate attacks since Aug. 22 in the gulf, which connects the Red Sea and the Indian Ocean and is one of the world's busiest waterways with some 20,000 ships passing through it each year.