Nov. 17 (Bloomberg) -- Somali pirates hijacked a Saudi Arabian-owned supertanker off the east coast of Africa, a sign of the increasing brazenness of attackers in one of the world's busiest waterways.

The Saudi Arabian Oil Co. tanker, Sirius Star, was carrying crude oil when pirates scaled the 10-meter (32-foot) side of the ship, Lieutenant Nate Christensen of the U.S. Navy's Fifth Fleet said by telephone from Bahrain. The vessel was last tracked on Nov. 10 leaving the Persian Gulf, bound for St. Eustatius in the Caribbean Sea, according to data compiled by Bloomberg.

It's the first seizure of a so-called very large crude carrier, the biggest used to transport oil. It's also the farthest pirate attack from shore, at about 450 nautical miles, Christensen said. Piracy in the Gulf of Aden, between Yemen and Somalia, has more than doubled in 2008 to at least 60 incidents, according to an October report by London-based Chatham House.

``It's quite an escalation,'' said Tudor Ellis, a maritime security expert at Drum Cussac, a London-based risk advisory company. ``They've taken chemical tankers before and they've attacked oil tankers, but they've never taken an oil tanker before.''

Crude Prices

About 11 percent of the world's seaborne petroleum passes through the Gulf of Aden en route to the Suez Canal or regional refineries. Crude prices briefly rose on the news.

The European Union last month joined the North Atlantic Treaty Organization, India, Malaysia and Russia in deploying vessels to combat piracy.

The tanker was heading to a terminal in St. Eustatius operated by San Antonio-based NuStar Energy LP, said Greg Matula, a company spokesman, in an e-mail.

Dhahran-based Saudi Arabian Oil, known as Saudi Aramco, declined to comment on the incident, and Vela International, Aramco's shipping unit, couldn't immediately be reached for comment by phone or e-mail.

Fifth Fleet Commander Jane Campbell said the incident took place southeast of Mombasa, Kenya, and that the ship was approaching anchorage near Somalia's northern semi-autonmous region of Puntland. She denied reports from Al-Arabiya television that it had been freed.

`Quite an Escalation'

The Sirius Star was seized at around 7:20 a.m. local time on Nov. 15, Cyrus Mody, an official at the International Maritime Bureau, said by phone, adding the IMB had no further information. The bureau compiles data on piracy.

Sirius Star, designed to carry more than 2 million barrels of crude, is three times the size of a U.S. aircraft carrier. Its seizure shows how pirates are successfully expanding their operations, Christensen said, adding that previous attacks have occurred within 200 miles of land.

The attack demonstrates ``the pirates' ability to adapt their tactics and methods of attack,'' said Vice Admiral Bill Gortney, the commander of the U.S. Navy's Combined Maritime Forces.

Still, the rate of successful pirate attacks on merchant vessels off the Somali coast has actually fallen in the past few months, to 31 percent in October from 53 percent in August, Gortney said in a statement.

Attack Thwarted

The increased international naval presence in the area has helped lower the rate, he said. For example, on Nov. 11, a U.K. warship ``successfully thwarted a pirate attack on a Danish shipping vessel and boarded the pirate ship responsible'' for the assault, Gortney said.

Companies operating merchant ships in the area could help the situation by placing their own security personnel on the ships, he said.

``Companies don't think twice about using security guards to protect their valuable facilities ashore,'' Gortney said. ``Protecting valuable ships and their crews at sea is no different.''

The attacks may force shipping away from the Gulf of Aden to take the longer route to Europe and North America around South Africa's Cape of Good Hope. The extra weeks of sailing and fuel consumption could increase oil and commodity prices, according to Chatham House.

Odfjell SE, the world's largest chemicals shipping line, said its vessels sail around Africa rather than risk attacks near the Suez Canal and Gulf of Aden.

The Sirius Star's crew of 25 includes citizens of Croatia, the U.K., the Philippines, Poland and Saudi Arabia, Christensen said.

`Probably Opportunistic'

The attack was ``probably opportunistic,'' Mark Jenkins, an analyst at London-based Simpson, Spence & Young Ltd., the world's second-largest shipbroker, said by phone. Failure to increase the protection of its fleet would probably ``compromise'' Saudi Aramco's ability to sell oil to customers in the U.S. and Europe, Jenkins said.

``Even if you've got to spend quite a lot of money, it's going to be a worthwhile investment if the alternative is you can't sell the oil as readily as you would otherwise aim to do,'' he said.

Crude oil for December delivery rose as high as $58.98 a barrel immediately after the report, recovering from an earlier low of $55.29 on the New York Mercantile Exchange. It traded at $55.49 at 2:23 p.m. New York time.

To contact the reporters on this story: Caroline Alexander in London at calexander1@bloomberg.net; Alaric Nightingale in London at Anightingal1@bloomberg.net

Last Updated: November 17, 2008 14:29 EST