Andrew Mwangura, the East African Coordinator of the Seafarers Assistance Program, said the oil tanker is the largest ship pirates have hijacked along the east Africa coast.

"It seems the vessel was hijacked on Saturday because the ship is approaching anchorage off the port of Eyl in Somalia. For it to reach there, it must have taken three to four days," Mwangura told Xinhua by telephone.

Reports from the US Navy said the tanker, Sirius Star, operated by Vela International, was hijacked after a group of pirates managed to scale the 10-meter high side of the ship.

Lieutenant Nate Christensen, a spokesman for the US Navy's Fifth Fleet said the hijacking was shocking because it 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.

The US Navy said the Saturday's hijacking of the oil tanker 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.

Sirius Star, designed to carry more than two million barrels of crude, "is three times the size of a US aircraft carrier and shows how they are successfully expanding their operations," Christensen said, noting that previous attacks have occurred within 200 nautical miles of land.

"We have heard reports that the ship has been freed and we are checking into it, we have no information to confirm."

Christensen said the bandits were taking it to an anchorage off the Somali port of Eyl. The port on Somalia's northeastern coast has become a pirate haven and a number of ships are already being held there as pirates try to force ransoms.

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 waters off Somalia's coast are considered to be some of the world's most dangerous -- pirates have hijacked more than 30 ships this year and attacked many more.

Most attacks have been in the Gulf of Aden between Yemen and north Somalia, a major route leading to the Suez Canal linking Europe and Asia.