Nairobi/Washington  - Somali pirates were Thursday morning holding the captain of a US-operated vessel hostage on a lifeboat as a dramatic standoff between the pirates, the crew of the vessel and a US Navy destroyer continued.

Pirates on Wednesday seized the Maersk Alabama, with 20 US citizens on board. It was the first time a ship with a US crew had been seized in the pirate-infested waters off Somalia.

However, the crew quickly fought back and regained control of the ship, capturing a pirate in the process.

Attempts to swap the pirate for Captain Richard Phillips - who is being held in the Maersk Alabama's lifeboat by several pirates - failed.

Second mate Ken Quinn earlier told broadcaster CNN that the crew was in touch with the captain, who had a ship radio.

The USS Bainbridge, part of a coalition naval force sent to combat piracy in the region, arrived early Thursday morning to assist the crew.

The crew of the USS Bainbridge was believed to be negotiating with the pirates, although the US Navy Fifth Fleet refused to comment.

US Navy forces are reluctant to storm ships to free crew members being held hostage, instead concentrating on preventative measures.

Owned by Danish firm Maersk, the 17,000-ton vessel, which was carrying food aid to Mombasa, Kenya, was taken in the Indian Ocean, around 500 kilometres off the Somali coast.

"This was going to Africa for people who are in need," John Reinhart, chief executive of Maersk Line, told reporters on Wednesday from the company's headquarters in Norfolk, Virginia.

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

The 32,000-ton British-owned Malaspina Castle, flying a Panama flag, was seized on Monday along with its crew of 24 from Bulgaria, Russia, Ukraine and the Philippines.

Taiwanese fishing vessel MV Win Far was taken on the same day near the Seychelles. Its crew of 30 is from Taiwan, China, Indonesia and the Philippines.

A French yacht, a Yemeni tugboat and a German container ship were also seized over the weekend.

Pirate gangs in 2008 seized dozens of vessels and earned tens of millions of dollars in ransoms, prompting the international community to hurriedly send warships to the region.

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

However, the pirates are now going further, venturing into the Indian Ocean off the southeast coast of Somalia, to avoid the patrols.

"In the east and south, there are no patrols, so if pirates attack ships there, the vessels are on their own," Noel Choong, head of the International Maritime Bureau's Piracy Reporting Centre, told German Press Agency dpa. "That is why we have seen hijackings in the last few days."(dpa)