Washington  - Crew members retook control of a US-flagged cargo ship that was hijacked off the coast of Somalia on Wednesday, but the ship's captain was still being held hostage.

Captain Richard Phillips was being held by the pirates on the ship's 28-foot lifeboat and was alive, second mate Ken Quinn told broadcaster CNN.

Quinn confirmed that the crew was in control of the ship and was waiting for additional support from a US Naval destroyer, which was a few hours away.

"We took one of the pirates hostage. We tied him up and kept him for 12 hours. We returned him, but they didn't return our captain," Quinn said.

"They're (pirates) not aboard. We're in control of the vessel. We can hear our captain - he's got a ship radio."

The ship with 20 US citizens on board was hijacked by Somali pirates early Wednesday morning, the first time a US crew has been taken hostage in the volatile waters.

Andrew Mwangura of the East African Seafarers' Association said that the 17,000-ton vessel, the Maersk Alabama, was taken in the Indian Ocean, around 500 kilometres off the Somali coast.

Four pirates apparently boarded the ship and at least one of them had been taken into custody by the crew, Defence Department officials said according to the American Forces Press Service.

The US State Department would not confirm that the ship was recaptured. Spokesman Robert Wood said there were still a number of "contradictory reports" coming out of the region.

Owned by Danish firm Maersk and operated by US company Maersk Line Limited, the ship was carrying food supplies and no weapons, and was headed to Mombasa, Kenya, according to John Reinhart, chief executive of Maersk Line.

"This was going to Africa for people who are in need," Reinhart told reporters from the company's headquarters in Norfolk, Virginia.

Reinhart said the crew had been trained to deal with attacks from pirates but did not carry weapons.

The White House earlier said it was "closely monitoring" the situation. A spokesman for the US Navy's Fifth Fleet stationed in Bahrain said 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 weeks. The Danish- US ship is the sixth to be 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.

The pirates do not harm their crew, instead holding out for multi- million dollar ransoms.

Pirate gangs in 2008 seized dozens of ships and earned tens of millions of dollars, prompting the international community to send in a fleet of warships.

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.

The warships, along with unfavourable weather conditions, appeared to have been deterring the pirates, with successful attacks dropping off between December and February.

However, the recent hijackings, many of which have been further out to sea, show that the pirates are changing tactics to avoid the patrolling vessels.

Despite the setbacks, however, there have been recent successes.

The German frigate Rhineland-Pfalz docked in the Kenyan port of Mombasa on Wednesday, carrying seven Somali pirates captured during an abortive attack on a German naval tanker in the Gulf of Aden last month.

The German Foreign Ministry said Kenya had taken custody of the pirates and they would appear before a judge on Thursday to determine whether they remain in detention.

Prosecuting pirates after they have been captured has proven a problem, with the law covering international waters unable to cope adequately with the issue. (dpa)