Somali pirates have hijacked a supertanker off the Horn of Africa, seizing the Saudi-owned ship loaded with crude oil and its 25-member crew, the U.S. Navy said Monday. The pirates were taking the ship to Eyl, a Somali port that has become a haven for seized ships, the navy spokesman said.The hijacking of the MV Sirius Star, one of the largest ships to sail the seas, occurred in the Indian Ocean far south of the zone patrolled by international warships in the busy Gulf of Aden shipping channel, which leads to and from the Suez Canal. It was the largest ship pirates have seized, and the farthest out to sea they have successfully struck.The hijacking Saturday highlighted the vulnerability of even very large ships and pointed to widening ambitions and capabilities among ransom-hungry pirates who have carried out a surge of attacks this year off Somalia.Maritime security experts said they have tracked a troubling spread in pirate activity southward into a vast area of the ocean that would be extremely difficult and costly to patrol, and this hijacking fits that pattern."It is very alarming," said Cyrus Mody, manager of the International Maritime Bureau. "It had been slightly more easy to get it under control in the Gulf of Aden because it is a comparatively smaller area of water which has to be patrolled, but this is huge."The tanker, owned by the Saudi oil company Aramco, is 330 meters, or about 1,100 feet, long, or the length of an aircraft carrier, and can carry about 2 million barrels of oil. Fully loaded, the ship's cargo could be worth about $100 million. But the pirates would have no way of selling the oil or refining it in Somalia.Lieutenant Nathan Christensen, a spokesman for the U.S. Navy's5th Fleet, said the Sirius Star was carrying crude at the time of the hijacking, but he did not know how much.There were no reports of damage to the ship, Christensen said. He declined to say if the U.S. Navy was considering taking action to rescue the tanker.The ship was sailing under a Liberian flag and its 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 vessel.The Sirius Star was attacked more than 450 nautical miles southeast of Mombasa, Kenya, the U.S. 5th Fleet said. The capturing of the oil tanker represents a "fundamental shift in the ability of pirates to be able to attack merchant vessels," Christensen said.