A Saudi supertanker hijacked by pirates with a $100 million oil cargo in the largest ever such seizure has reached the coast of north Somalia, a regional maritime group said on Tuesday.

"Some people are saying they have spotted a huge vessel off Eyl. It must be the supertanker," Andrew Mwangura, coordinator of the East African Seafarers' Association, told Reuters, referring to a remote coastal village used by Somali pirates.

The  pirates had captured the fully laden Saudi supertanker Sirius Star far off east Africa.

The hijacking of the Saudi Aramco-owned vessel on Sunday is certain to add to pressure for concerted international action to tackle the growing threat posed by pirates from anarchic Somalia to one of the world's busiest shipping routes.

"This is unprecedented. It's the largest ship that we've seen pirated," said Lt Nathan Christensen, a spokesman for the Fifth Fleet. "It's three times the size of an aircraft carrier."    

The Sirius Star held as much as two million barrels of oil -- more than one quarter of Saudi Arabia's daily exports. The hijacking helped lift global oil prices over $1 to more than $58 a barrel, although they later lost some gains.

The hijacking on Sunday, 450 nautical miles (830 km) southeast of Mombasa, Kenya, was in an area far beyond the Gulf of Aden, where most of the attacks on shipping have taken place and where foreign navies have begun patrols.

Navy Adm Mike Mullen, chairman of the US Joint Chiefs of Staff, suggested military intervention would be complicated by hostages and ransom demands.

"I'm stunned by the range of it," said Mullen, telling reporters at the Pentagon that distance from the African coast was the longest he had seen to date.

"Once they get to a point where they can board, it becomes very difficult to get them off because, clearly, now they hold hostages."    

The Sirius Star had been heading for the United States via the Cape of Good Hope at the southern tip of Africa, skirting the continent instead of heading through the Gulf of Aden and then the Suez Canal.

The ship, at 318,000 deadweight tons, was the largest ever captured by pirates. - Reuters