The Greece-flagged Maran Centaurus was hijacked Sunday about 800 miles (1,300 kilometers) off the coast of Somalia, said Cmdr. John Harbour, a spokesman for the EU Naval Force. Harbour said it originated from Jeddah, Saudi Arabia and was destined for the United States. The ship has 28 crew members on board, he said.
The shipping intelligence company Lloyd's List said the Maran Centaurus is a "very large crude carrier, with a capacity of over 300,000 tons." Officials could not immediately say how many barrels of oil were on board, but its value would be in the millions of dollars.
Pirates have increased attacks on vessels off East Africa for the millions in ransom that can be had. Though pirates have successfully hijacked dozens of vessels the last several years, Sunday's attack appears to be only the second ever on an oil tanker.
The hijacking of a tanker increases worries that the vessel could crash, be run aground or be involved in a firefight, said Roger Middleton, a piracy expert at London-based think tank Chatham House.
Pirates typically use guns and rocket-propelled grenades in their attacks, and some vessels now carry private security guards, but Middleton said oil tankers do not.
"You're sitting on a huge ship filled with flammable liquid. You don't want somebody with a gun on top of that," Middleton said. "Financially it's a very costly exercise because the value of oil is so volatile. If it is held for a long time and the price of oil drops, they could lost millions of dollars."
In November 2008, pirates hijacked the Saudi supertanker Sirius Star, which held 2 million barrels of oil valued at about $100 million. The tanker was released last January for a reported $3 million ransom after a two-month drama that helped galvanize international efforts to fight piracy off Africa's coast.
Somali pirates are a separate group of criminals from the al-Qaida-affiliated Islamic militants who control large areas of southern Somalia, but anytime pirates hold such valuable and explosive cargo it raises international concerns.
In late 2007, pirates hijacked a chemical tanker carrying up to 10,000 tons of highly explosive benzene. Initially, American intelligence agents worried terrorists from Somalia's Islamic extremist insurgency could be involved, and might try to crash the boat into an offshore oil platform or use it as a gigantic bomb.
When the Japanese vessel was towed back into Somali waters and ransom demanded, the coalition was relieved to realize it was just another pirate attack.
Somalia's lawless 1,880-mile (3,000-kilometer) coastline provides a perfect haven for pirates to prey on ships heading for the Gulf of Aden, one of the world's busiest shipping routes. The impoverished Horn of Africa nation has not had a functioning government for a generation and the weak U.N.-backed administration is too busy fighting the Islamist insurgency to arrest pirates.
Pirates now hold about a dozen vessels hostage and more than 200 crew members. The Maran Centaurus had 28 crew aboard - 16 Filipinos, nine Greeks, two Ukrainians and one Romanian, Harbour said.
Middleton said pirate demands and negotiations are becoming more complex.
"They still want the money but they have also asked for the release of imprisoned comrades," he said. "That demand is an extra bargaining tool they can use to add extra layers to their negotiating position."
Piracy has increased despite an increased presence by international navies patrolling the Indian Ocean and Gulf of Aden. The U.S. this fall began flying sophisticated drones over East African waters as part of the fight against piracy.
Associated Press Writer Katharine Houreld contributed to this report.