PIRATES from Somalia have seized the biggest booty ever taken on the high seas, capturing a fully laden Saudi oil supertanker and its multinational crew.

The brand new Sirius Star - three times the size of an aircraft carrier and carrying its full complement of two million barrels of crude oil worth at least $US100million ($154million) - was hijacked in the early hours of Sunday, 450 nautical miles southeast of Mombasa, Kenya, according to the US Fifth Fleet.

"This is unprecedented," fleet spokesman Lieutenant Nathan Christensen said yesterday.

"It's the largest ship we've seen pirated."

The pirates punctured a security web of warships protecting vital shipping lanes, highlighting the expanding reach of the well-armed fighters, bolstered by millions from past ransoms.

With most attacks ending after million-dollar payouts, piracy is considered the biggest economy in Somalia. Pirates have raked in up to $US30million in ransoms this year.

The pirates are trained fighters, often dressed in military fatigues, using speedboats equipped with satellite phones and GPS equipment. They are typically armed with automatic weapons, rocket-launchers and grenades.

Last night, the Sirius Star was reported to have anchored off the port of Haradhere, a village and piracy hub, 300km north of the Somali capital of Mogadishu. There were fears of an environmental catastrophe if the pirates run aground in waters far too shallow for the vast supertanker.

Shipping analysts said the cost of sending freight around the world would rise following the attack as a result of higher insurance premiums and an increase in charter rates.

The Sirius Star is the latest of more than 60 vessels to be captured off the Somali coast this year, but the first supertanker. Jitters over the ease with which pirates seized crude equivalent to a quarter of Saudi Arabia's daily output sent falling oil prices into reverse yesterday. They finished up $US1 per barrel.

Odfjell, one of the largest shipping groups in the world, responded to the attack by suspending its routes through the Gulf of Aden in favour of the longer journey around the Cape of Good Hope at the tip of South Africa, raising the prospect that one of the world's busiest trade routes could be sidelined unless global action was taken to combat the pirate menace.

Britain leads a multinational taskforce in the area. Last week, the Royal Navy was drawn into a shootout with a gang attempting to hijack a cargo ship, killing two of the pirates.

But the capture of the Sirius Star hundreds of kilometres to the south in the Indian Ocean, as it was heading to the US via the Cape of Good Hope, suggests the Gulf pirates are simply moving into unpatrolled waters or that other pirate groups, recently dormant, have been reawakened. The supertanker had avoided the Gulf of Aden and the Suez Canal because it was too big to pass through the canal. It is not only the largest ship to be captured, but the farthest from the Somali shoreline.

The top US military officer, Michael Mullen, said he was astounded by the reach of the Somali pirates. "I'm stunned by the range of it, less so than I am the size," Admiral Mullen said.

The US Fifth Fleet declined to say whether military action was being considered to rescue the giant tanker, which has a crew of 25 from Croatia, The Philippines, Poland, Saudi Arabia and Britain.

Shipping experts said a rescue attempt was unlikely because of the extreme danger to the crew and the ship.

Vela International, which operates the tanker for the Saudi state oil company Saudi Aramco, said it had set up a negotiating team to deal with ransom demands.

"All 25 crew members on board are reported to be safe," the company said.

"Vela response teams have been established and are working to ensure the safe release of the crew members and the vessel."

Somalia has lacked a functioning government since the outbreak of civil war in 1991. But the lawlessness that has prevailed since the ousting of the Islamic Courts government last year has spawned the epidemic of piracy.

The gangs' methods vary little, even when they are taking a 320,000-tonne monster such as the Sirius Star. Gunmen typically approach on small speedboats, opening fire on the bridge until the ship's captain submits and allows them on board, usually throwing down a ladder.

The average reaction time between spotting the pirates and being boarded is 15 minutes. Crews are strictly instructed not to resist attack when arms are used. Once captured, violence against crew members is rare.

In recent months, the pirates' arsenal has grown more deadly, with rocket-propelled grenade-launchers and possibly shoulder-mounted missiles used to threaten the crew.

And the pirate groups have hugely extended their reach from the coast with the use of "mother ships" - larger vessels from which they launch speedboats after they have identified their potential prey.

While some mother ships have been identified, other attacks are launched from ordinary dhows -- traditional sailing boats hijacked from fishermen.

Negotiations with ships' owners can go on for several months and are clouded in secrecy. At least 12 ships with more than 250 crew members are being held as negotiations continue. Among them is the Ukrainian arms ship Faina, which was captured in September with a cargo of 33 battle tanks, hundreds of crates of Kalashnikovs and ammunition.

Shipping companies have noticed a pattern in which new hijacks occur within days of a ransom settlement, suggesting that the gangs move from one hijack to another as soon as the last is resolved.

"There are never less than 10-12 ships being held," said Giles Knowles, head of maritime security for the Baltic and International Maritime Council, which represents more than 2700 of the world's shipping companies.

The past week had seen the resolution of three ransom settlements, he said, with three more hijacks promptly taking place since Friday. "It would seem there is a cycle."

The Faina and its 20-member crew are still being held off Somalia, watched by warships to prevent the removal of its cargo.

The seizure of the Faina and its cargo of weapons prompted a reinforcement of warships patrolling the waters off Somalia. Along with a Russian frigate and Indian vessels, a NATO flotilla of seven ships is in the Gulf of Aden to help the US 5th Fleet in anti-piracy patrols and to escort cargo vessels. The 5th Fleet said it had repelled about two dozen pirate attacks since August 22 in the gulf.

Another multinational fleet led by the Dutch has carved out a protected lane through the Gulf of Aden, through which 20,000 tankers, freighters and merchant vessels transit every year, entering and exiting the Suez Canal.

Odfjell chief executive officer Terje Storeng said yesterday: "We will no longer expose our crew to the risk of being hijacked and held for ransom by pirates in the Gulf of Aden.

"The rerouting will entail extra sailing days and later cargo deliveries," he said. "This will incur significant extra cost, but we expect our customers' support and contribution."

Mr Knowles said several companies had already consulted Bimco - an independent international shipping association of shipowners, managers, brokers and agents - on moving to the Cape route, extending the average journey by three weeks, and that he expected more to take Odfjell's lead.

Bimco has called on foreign governments to send more warships in the short term to work under a UN mandate to police the Somali coast. In the longer term, it would like a permanent UN coastguard force.

But as long as anarchy reigns onshore, little will change at sea. "Historically you've never defeated piracy at sea," Mr Knowles said. "The resolution lies ashore - in Somalia."

The Times, AP