Somali pirates hijacked a giant Saudi supertanker with a cargo of crude oil hundreds of miles out in the Indian Ocean in a dramatic escalation that showed their expanding reach, puncturing a security web of warships trying to protect vital shipping lanes. It was the largest vessel seized yet in a surge of pirate attacks, and the farthest out to sea that the well-armed fighters, bolstered by millions in past ransoms, have successfully struck. Maritime experts warned that the broad daylight attack, reported by the US Navy on Monday, was an alarming sign of the difficulty of patrolling a vast stretch of ocean key for oil and other cargo traffic. The brand new MV Sirius Star tanker, with a 25-member crew, was seized at about 10 a.m. Saturday more than 450 nautical miles southeast of Mombasa, Kenya, the US Navy said. The area lies far south of the zone where warships have increased their patrols this year in the Gulf of Aden, one of the busiest channels in the world, leading to and from the Suez Canal, and the scene of most past attacks. The massive supertanker would seem to present a daunting target for the pirates, who usually operate in small speedboats. At 330 meters (1,080 feet), it's the length of an aircraft carrier, able to carry about 2 million barrels of oil.