The US Navy on Friday encircled Somali pirates holding an American hostage on a lifeboat who desperately tried to muster their own reinforcements.

Four pirates on Wednesday hijacked the Maersk Alabama aid ship before being overpowered by the unarmed American crew and ousted from the 17 500-ton Danish-operated container ship.

But in a twist to the high-seas drama, the pirates took the captain hostage on a lifeboat and the US Navy and the US Federal Bureau of Investigation are trying frantically to secure his release.

More US ships, including from a counter-piracy task force out of Bahrain, were on the way to join the destroyer USS Bainbridge that arrived on Thursday to take position against the pirates, defence officials said.

The Bainbridge, accompanied by a P-3 Orion surveillance plane, was preventing the pirates from moving their hostage to a larger ship.

"The safe return of the captain is the top priority," Defense Secretary Robert Gates told reporters in Washington.

Secretary of State Hillary Clinton said the pirates' lifeboat was "apparently" out of fuel though military officials declined to confirm her account.

A commander from the gang of Somali pirates who took the ship said more pirates were on their way.

"We are planning to reinforce our colleagues who told us that a navy ship was closing in on them and I hope the matter will soon be solved," Abdi Garad said by phone from the northern pirate lair of Eyl.

"They are closely monitored by a navy ship and I think it will be difficult for us to reach the area promptly. But we are making final preparations and will try our best to save our friends."

Kevin Speers, a spokesperson for the shipowners Maersk, told reporters that "most recent contact with the Alabama indicated that the captain remains a hostage but is unharmed at this time."

Meanwhile the freighter was boarded by military personnel and headed to its destination port of Mombasa, in Kenya, with its cargo of aid destined for African refugees, US and company officials said.

It had been due to dock in Mombasa on April 16 to deliver more than 5000 tonnes of relief food supplies to the United Nations World Food Programme (WFP).

The Maersk Alabama's chief officer, Shane Murphy, reportedly told his father the crew used "brute force" to overpower the pirates, who were armed with AK-47 assault rifles.

With six hijackings in the space of four days, Somalia's pirates have dashed any hope that increasing naval presence in the region could significantly dent a scourge that is disrupting one of the world's busiest maritime trade routes.

Since the start of the year, piracy watchdogs had recorded a slump in the number of attacks and their success rate compared to 2008, during which pirates attacked close to 150 ships and harvested a bumper crop of ransom money.

Hans Tino Hansen, managing director of Denmark-based Risk Intelligence, said one of the main reasons for the sudden surge in attacks was simply the weather.

"Weather is king. Due to the profile of the pirates' skiffs and other vessels, they are very dependent on favourable weather conditions, which has been the case east of Somalia lately," he said.

This has allowed pirates to venture further out at sea and avoid the now heavily-patrolled shipping corridors in the Gulf of Aden, where maritime traffic bottlenecks in and out of the Red Sea.

Analysts and military officers say pirate attacks are likely to grow given the lucrative ransom money paid by shipping companies and the lawless nature of Somalia.

Since April 4, Somali pirates have hijacked a US container ship, a small French sailing yacht, a British-owned Italian-operated cargo, a German container carrier, a Taiwanese fishing vessel and a Yemeni tugboat.

Some of the pirates' most spectacular successes came late last year when they seized a Ukrainian cargo loaded with combat tanks and other weaponry, as well as a Saudi super-tanker carrying 100 million dollars in crude oil.

The combined ransoms paid for the release of these two ships alone is believed to be around eight million dollars (six million euros) and the pirates are known to significantly reinvest in better equipment.