Nairobi/Washington - Further US Navy ships were steaming Friday to the coast of Somalia to increase pressure on pirates holding the captain of a US-operated ship on a stranded lifeboat. The Maersk Alabama, a cargo ship carrying food aid, was seized by pirates Wednesday only for the crew to immediately retake the ship. However, Captain Richard Phillips ended up being held on the Alabama's lifeboat by the pirates.

The USS Bainbridge, part of a coalition naval force sent to combat piracy in the region, arrived early Thursday and made contact with the lifeboat, according to a spokesman for Maersk Line Ltd, the US company that operates the ship.

Additional Navy ships were being sent to the region and were expected to arrive within 48 hours, General David Petraeus, who commands US forces in the Middle East, said Thursday in Florida.

One of the ships is believed to be the USS Halyburton, a frigate with two helicopters on board.

It remained unclear how Phillips came to be held captive on the lifeboat although some media reports suggested he exchanged himself for the safety of his crew.

"The captain remains with the pirates on the lifeboat within full visibility of the USS Bainbridge," Maersk Line spokesman Kevin Speers said in a statement.

Phillips has been in contact with the US Navy and his own crew through a radio and has been given extra batteries. The US Federal Bureau of Investigation is also helping the Navy negotiate the captain's release.

The Alabama steamed away from the area and was heading Friday toward its original destination of Mombasa, Kenya.

Some observers said they expect the hostage drama to drag on for days. Navy forces are generally reluctant to storm ships to free crew members being held hostage.

However, the pirates are in a weak bargaining position with no fuel for the lifeboat and only one hostage.

US Defence Secretary Robert Gates told reporters in Washington that the "safe return of the captain is the top priority."

Somali pirates have stepped up their attacks in recent weeks after a brief lull. The Maersk Alabama was the sixth ship to have been seized since Saturday.

Pirate gangs in 2008 seized dozens of vessels in and around the Gulf of Aden and collected tens of millions of dollars in ransoms, prompting the international community to send warships to the region.

Around 15 warships from the European Union, a coalition task force and individual countries such as Russia, the United States, India and China patrol an area of about 2.85 million square kilometres.

However, the pirates are now venturing farther into the Indian Ocean off the south-east coast of Somalia to avoid the international patrols.

The 17,000-ton Alabama, owned by the Danish firm Maersk, was taken around 500 kilometres off Somalia.

Many observers said they feel piracy can only be stopped by dealing with insecurity on the ground in Somalia. A bloody insurgency is ongoing in south and central Somalia, which has not had a functioning government since 1991.

US Secretary of State Hillary Rodham Clinton said the military was working with its allies to see how to develop a broader plan to deal with piracy in the region.

"Piracy may be a centuries-old crime, but we are working to bring an appropriate 21st-century response," she said. (dpa)