Piracy has made Somali waters among the world's most dangerous
A South Korean navy warship is in pursuit of a huge oil tanker, hijacked by Somali pirates in the Indian Ocean.
The 300,000-tonne Samho Dream, which was on its way from Iraq to the United States, has 24 crew on board, and is loaded with crude oil.
Reports suggest the Korean destroyer is fast enough to catch up to the tanker before it reaches the Somali coast.
Pirates targeting ships off the coast of Somalia made tens of millions of dollars in ransom payments last year.
South Korea is one of several Asian nations that have an anti-piracy warship patrolling Somali waters to guard against hijackings. Western navies are also trying to protect ships against pirate attack.
The destroyer now in pursuit of the South Korea-operated, Singapore-owned tanker was on anti-piracy patrol in the Gulf of Aden - one of the world's busiest and most dangerous shipping lanes.
It has been diverted some 1,500 km (930 miles) south-east of the Gulf to the area where the hijacking took place.
A South Korean official said the destroyer had been ordered to intercept the hijacked vessel on its expected route into Somali waters, according to Yonhap news agency.
He also expressed concern for the safety of the crew - five South Koreans and 19 Filipinos - but said the government would not negotiate with the pirates.
It is unclear what action the warship will take once it reaches the tanker.
The BBC's East Africa correspondent Will Ross says that it is extremely rare for any navy to use force once hostages have been taken.
Given the nature of the cargo there is also the risk of immense environmental damage, he adds.
The value of the Samho Dream's cargo is estimated at about $170m (£111.7m).
Reuters reported that the US refiner Valero Energy Corp said it was the owner of the crude oil cargo.
It said a pirate source named Mohamed had said the ship was heading for Haradheere, the pirates' base at which many ships are held during ransom negotiations.
At least four South Korean ships have been hijacked by Somali pirates in recent years: a tuna ship with 25 crew in 2006, two ships and 24 crew (held captive for six months) in 2007, and a cargo ship with 22 sailors in September 2008.
The crew in that last attack were released after the ship's owner paid a ransom.
The first successful hijacking of a so-called Very Large Crude Carrier was of the Saudi-owned Sirius Star in late 2008.
Another VLCC, the Maran Centaurus, was taken last November and held for two months before a ransom estimated at between $5.5m and $7m was paid.