Pirates have seized a Saudi-owned supertanker and its 25 crew members off the Kenyan coast, the United States Navy announced Monday.

"Increasingly daring attacks are being conducted by Somali pirates on a variety of merchant vessels," said a statement from the U.S. Fifth Fleet, issued from Manama, Bahrain.

The statement said that pirates attacked the very large crude tanker, Sirius Star, more than 450 nautical miles southeast of Mombasa. The ship was sailing under a Liberian flag and was owned by the Saudi-based company, Saudi Aramco. The crew comprised citizens of Croatia, Britain, The Philippines, Poland and Saudi Arabia.

Last week a British military team from the frigate HMS Cumberland killed two suspected pirates in a shoot-out with the occupants of a dhow which the Royal Navy said had been identified as having earlier been involved in an attempt to seize a Danish vessel.

The U.S. Navy statement said that although the stepped-up naval presence off the coast of Somalia had reduced successful attacks on shipping, naval forces could not be everywhere.

"Our presence in the region is helping deter and disrupt criminal attacks off the Somali coast," said Vice Admiral Bill Gortney, commander of Combined Maritime Forces, "but the situation with the Sirius Star clearly indicates the pirates' ability to adapt their tactics and methods of attack."