The Sirius Star made its maiden voyage in March of this year
Somali pirates have seized a giant Saudi-owned oil tanker in the Indian Ocean and are steering it towards Somalia, the US Navy reports.
The Sirius Star is the biggest ship ever to be hijacked, with a capacity of 2m barrels - more than one-quarter of Saudi Arabia's daily output.
The vessel was captured on Saturday some 450 nautical miles (830km) off the Kenyan coast.
Its international crew of 25, including two Britons, is said to be safe.
US Navy officials said the hijacking was unprecedented and marks a fundamental shift in their capabilities.
Admiral Mike Mullen, chairman of the US Joint Chiefs of Staff, said the pirates were well trained.
"They're very good at what they do," he told a Pentagon briefing in Washington.
"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 US Navy said the ship was "nearing an anchorage point" at Eyl, a port often used by pirates based in Somalia's Puntland region.
War-torn Somalia has not had an effective government since 1991.
News of the attack raised crude oil prices on global markets following an earlier slump, Reuters news agency reported.
The capture, south-east of Mombasa, is highly unusual both in terms of the size of the ship and the fact it was attacked so far from the African coast, BBC security correspondent Frank Gardner says.
He says that such hijackings are usually resolved peacefully through negotiations but, given the high profile of this event and the value of the cargo, there is always the possibility of some kind of military response.
Attacks on shipping off the Horn of Africa and Kenya by mainly Somali pirates seeking ransoms prompted foreign navies to send warships to the area this year.
A number of vessels and their crews have been held captive for months, including the Ukrainian freighter MV Faina, seized in September.
The supertanker was heading for the US via the Cape of Good Hope at the southern tip of Africa, Reuters reports.
THE SIRIUS STAR
Nearly the length of a US aircraft carrier
Weighs more than three times as much as a carrier when loaded
Can carry 2 million barrels of oil - more than 25% of Saudi Arabia's daily output
Is third tanker, and biggest vessel, to be hijacked in the region
The route around the Cape of Good Hope is a main thoroughfare for fully-laden supertankers from the Gulf.
With a capacity of 318,000 dead weight tonnes, the ship is 330m (1,080ft) long and is classed as a Very Large Crude Carrier.
It is about as long as a US aircraft carrier and, when loaded, weighs more than three times as much.
"It's the largest ship that we've seen pirated," said Lt Christensen.
The South Korean-built Sirius Star, owned by the Saudi company Aramco, made its maiden voyage in March 2008.
The ship's operator, Vela International, said that all of the crew were reported to be safe.
"Vela response teams have been mobilised and are working to ensure the safe release of crew members and the vessel," it added.
Confirming that two Britons were aboard the tanker, the UK Foreign Office said it could not give any details of their role on the ship.
"We are seeking more information on the incident," a spokesman said.
The other crew are said to be from Croatia, the Philippines, Poland and Saudi Arabia.
Figures from the International Maritime Bureau show that attacks in the area - the Gulf of Aden and the Indian Ocean off the African coast - have made up one-third of all piracy incidents worldwide in 2008.
In the first nine months of the year 63 incidents were reported.
As of 30 September, 12 vessels remained captive and under negotiation with more than 250 crew being held hostage.
Pirates remain active and regularly strike in the region. In the past week alone:
• A Russian warship in the Gulf of Aden drove off pirates who tried to capture the Saudi Arabian merchant ship Rabih
• Pirates hijacked a Japanese cargo ship off Somalia
• A Chinese fishing boat was seized off the Kenyan coast
• A Turkish ship transporting chemicals to India was hijacked off Yemen
• The UK's Royal Navy shot dead two suspected pirates attacking a Danish cargo-ship off the coast of Yemen