Washington  - The United States is considering its options to assure the safety of 21 crew members being held after pirates hijacked a US-operated vessel off the coast of Somalia, the White House said Wednesday.

The White House is closely monitoring the apparent hijacking of the US-flagged ship in the Indian Ocean and assessing a course of action to resolve this situation," spokesman Robert Gibbs said in a statement.

"Our top priority is the personal safety of the crew members on board," Gibbs said.

The ship, owned by Danish firm Maersk and operated by US company Maersk Line Limited, was taken Wednesday morning and is believed to be headed for the Somali port town of Eyl.

The hijacking occurred near the US Navy's Fifth Fleet stationed in Bahrain. A Navy spokesman there said that a coalition force off the Somali coast was keeping watch on the vessel.

Somali pirates, who typically seek ransom, have stepped up their attacks on ships operating off the coast in recent months. The Danish-US ship is the sixth to be seized since Saturday.