The 318,000 deadweight tonnes Very Large Crude Carrier is the largest vessel to be seized in an epidemic of piracy in the region.
"According to the latest report we have, the ship is approaching the Somali coast, heading towards Eyl (port)," a spokeswoman for the Bahrain-based US Fifth Fleet told AFP by telephone from Dubai. "Can we assume that the pirates are Somalis? Yes."
Sirius Star, which is owned by Saudi Aramco, carried 25 crew members from Croatia, Britain, Philippines, Poland and Saudi Arabia, according to a US Navy statement.
The South Korea-built ship, launched earlier this year, is operated by Vela International and registered in Liberia.
Vela said the fully-laden tanker was seized by a group of armed men about 420 nautical miles off the coast of Somalia on Sunday.
"All 25 crew members on board are reported to be safe. Vela response teams have been established and are working to ensure the safe release of the crew members and the vessel," the company said.
The International Maritime Bureau has reported that at least 83 ships have been attacked off Somalia since January, of which 33 were hijacked. Of those, 12 vessels and more than 200 crew were still in the hands of pirates.
"Our presence in the region is helping deter and disrupt criminal attacks off the Somali coast, but the situation with the Sirius Star clearly indicates the pirates’ ability to adapt their tactics and methods of attack," Vice Admiral Bill Gortney said in the US Navy statement.
Last week, the European Union started a security operation off the coast of Somalia, north of Kenya, to combat growing acts of piracy and protect ships carrying aid agency deliveries. It is the EU’s first-ever naval mission.
Dubbed Operation Atlanta, the mission, endorsed by the bloc’s defence ministers at talks in Brussels, is being led by Britain, with its headquarters in Northwood, near London.
Pirates are well organised in the area where Somalia’s northeastern tip juts into the Indian Ocean, preying on a key maritime route leading to the Suez Canal through which an estimated 30 percent of the world’s oil transits.
They operate high-powered speedboats and are heavily armed, sometimes holding ships for weeks until they are released for large ransoms paid by governments or owners.
NATO warships, along with ships and aircraft from several other nations have been deployed in the region to protect commercial shipping.
Norwegian shipping company Odfjell said on Monday it will no longer sail through the pirate-plagued Gulf of Aden, choosing instead the longer, more expensive but also safer route around Cape of Good Hope.
Somalia has lacked an effective government since the 1991 ouster of president Mohamed Siad Barre touched off a bloody power struggle that has defied numerous attempts to restore stability.
