A giant Saudi oil tanker seized by pirates in the Indian Ocean is nearing the coast of Somalia, the US Navy says.
The Sirius Star is the biggest tanker ever to be hijacked, with a cargo of 2m barrels - a quarter of Saudi Arabia's daily output - worth more than $100m.
The vessel was captured in what the navy called an "unprecedented" attack 450 nautical miles (830km) off the Kenyan coast on Saturday.
Its international crew of 25, including two Britons, is said to be safe.
The ship's operator, Vela International, said a response team had been mobilised to work towards ensuring the safe release of vessel and crew
92 attacks this year
36 successful hijackings
14 ships currently held, including the MV Faina carrying tanks
268 crew held hostage
Source: International Maritime Bureau, 2008
Saudi Arabia's foreign minister, Prince Saud al-Faisal, has condemned the hijacking as "an outrageous act", comparing piracy to terrorism.
Speaking on Tuesday during a visit to Athens, he described piracy as "a disease which is against everybody, and everybody must address it together".
The hijacking was highly unusual both in terms of the size of the ship and the fact it was attacked so far south of Somalia, says BBC security correspondent Frank Gardner.
The seizure points to the inability of a multi-national naval task force sent to the region earlier this year to stop Somali piracy, he says.
The US Fifth Fleet said the supertanker was "nearing an anchorage point" at Eyl, a port often used by pirates based in Somalia's Puntland region.
Adm Mike Mullen, chairman of the US Joint Chiefs of Staff, said the pirates involved were well trained.
"Once they get to a point where they can board, it becomes very difficult to get them off, because, clearly, now they hold hostages," he told a Pentagon briefing in Washington.
Commander Jane Campbell, of the US Navy's 5th Fleet, told the BBC it had warned shipping companies that the US naval presence could "not be everywhere", adding: "For that reason we have strongly encouraged proactive self-protection measures for the companies."
Oil price rises
Hijackings off the coast of East Africa and the Gulf of Aden - an area of more than 1m sq miles - make up one-third of all global piracy incidents this year, according the International Maritime Bureau (IMB).
They are usually resolved peacefully through negotiations for ransom but, given the value of the cargo in this instance, a military response has not been ruled out, our correspondent says.
Fourteen vessels - including the Ukrainian freighter MV Faina carrying 33 tanks and other military hardware, which was seized in September - remain captive in Somalia and under negotiation with around 268 crew being held hostage, according to the IMB.
The group's Piracy Reporting Centre said it had records of 92 attacks against vessels so far this year, including 36 successful hijackings.
This month alone, pirates have seized a Japanese cargo ship off Somalia, a Chinese fishing boat off Kenya and a Turkish ship transporting chemicals off Yemen.
War-torn Somalia has not had an effective government since 1991 and the anarchy on land has spread to the high seas in recent years.
The South Korean-built Sirius Star was seized as it headed for the US via the southern tip of Africa, prompting a rise in crude oil prices on global markets.
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 - about the length of a US aircraft carrier.
Owned by the Saudi company Aramco, it made its maiden voyage in March.
As well as the two Britons, the ship's crew members are said to be from Croatia, the Philippines, Poland and Saudi Arabia.