The US Navy prepared on Friday to increase pressure on Somali pirates to give up an American ship captain held hostage in a lifeboat in a strategic part of the Indian Ocean for two days.

The standoff was sparked when four gunmen briefly hijacked the 17,000-tonne Maersk Alabama freighter on Wednesday in waters that are a busy shipping zone for oil tankers and other commercial vessels but infested with pirates.

The USS Bainbridge, a naval destroyer, was patrolling the area while FBI and other US officials attempted to negotiate with the pirates and persuade them to surrender Richard Phillips, the freighter's captain.

Phillips apparently volunteered to get in the lifeboat with the pirates, acting as a hostage for the Alabama's 20 American crew members, who retook control of the ship after a confrontation 500 km off the coast of Somalia.

US military officials said more forces were on the way and that all options were on the table to save the captain, a former Boston taxi driver who is now the first US citizen seized by Somali pirates.

'We're definitely sending more ships down to the area,' a defense official told Reuters. He said one of the ships would be the USS Halyburton, a guided missile frigate that has two helicopters on board.

The attack on the Alabama could lead to a new phase in the international struggle against piracy in the region, galvanizing political support in the United States for a more active US security presence on crucial trade routes off Africa's coasts.

FBI fully engaged

An upsurge in piracy has disrupted shipping in the strategic Gulf of Aden and busy Indian Ocean waterways, delayed delivery of food aid for drought-hit East Africa, increased insurance costs and made some firms send cargoes around South Africa instead of through the Suez Canal, a critical route for the oil trade.

Heavily armed sea gangs hijacked dozens of vessels last year, including a Saudi supertanker with $100 million of oil and a Ukrainian ship with 33 tanks. The Saudi and Ukrainian ships fetched about $3 million each.

Pirates hold 18 vessels with a total of 267 hostages, many of them from the Philippines, according to the Kenya-based East African Seafarers' Assistance Programme.

'Piracy may be a centuries-old crime, but we are working to bring an appropriate, 21st century response,' US Secretary of State Hillary Clinton said on Thursday, adding that 'a number of assets' were being used to resolve the Alabama situation.

The FBI said it had been called in to assist, and its negotiators were fully engaged in resolving what Attorney General Eric Holder called the first act of piracy against a US vessel 'in hundreds of years'.

In the face of the growing US response, the four pirates appear to have realized that they may have overplayed their hand.

Reached by Reuters via satellite phone, they sounded desperate. 'We are surrounded by warships and don't have time to talk,' one said. 'Please pray for us.'

The Maersk Alabama was sailing from Djibouti to Mombasa with a cargo of food aid for Somalia and Uganda when it was attacked.

In a statement the Danish-owned freighter's operator, Maersk Line, said the ship had since left the area for Kenya. It said latest communications indicated the captain was unharmed.

In Somalia's Haradheere port, an associate of the pirates said they were armed and ready to defend themselves.

'Our friends are still holding the captain, but they cannot move, they are afraid of the warships,' he said. 'We want a ransom and, of course, the captain is our shield. The warships might not destroy the boat as long as he is on board.'

Analysts said Somali pirates would not be eliminated or even deterred until there was a political solution in Somalia, which has endured chaos sinc