Nairobi/Washington - Somali pirates, holding a US ship captain in a lifeboat and surrounded by two Navy warships, have reportedly sought ransom and safe passage as their brazen standoff with the US military continued on Friday.

The pirates' demands came as a US frigate, the USS Halyburton, became the second US Navy vessel to arrive in the area off the Somali coast, according to Commander Peter Schneider, a spokesman for the US Defence Department.

The frigate joined the USS Bainbridge, a destroyer, which arrived Thursday morning and had taken up negotiations with the four pirates, who have been holed up in the lifeboat since an aborted attempt Wednesday to capture the Maersk Alabama, a US-operated cargo ship.

The US Federal Bureau of Investigation was also aiding the negotiations, yet the pirates have told the Navy they will not release Captain Richard Phillips without a ransom and guaranteed safe passage out of the area, according to Bloomberg News, which interviewed one of the pirates' contacts on the Somali mainland.

Phillips reportedly attempted to flee from his captives earlier Friday, leaping from the lifeboat in a daring attempt to swim to the Bainbridge.

The incident happened too quickly for the Bainbridge crew to help Phillips, who was apparently unhurt during his recapture. Schneider said he could not confirm the exact details of the escape attempt and the Navy had not been in contact with the captain since the incident.

Andrea Phillips, the captain's wife, said she has received an "outpouring of support" for her husband since the crisis began.

"We have felt the compassion of the world through your concern for Richard. My husband is a strong man and we will remain strong for him," she said in a statement released by Maersk Line Ltd, the Virginia-based company that operated the Alabama.

The Alabama, a cargo vessel carrying food aid, was boarded by the pirates Wednesday morning, the first time US sailors were seized in the treacherous waters near the Horn of Africa. The unarmed crew quickly retook the ship, but Phillips ended up being held on the Alabama's lifeboat by the pirates.

The Alabama has since steamed away from the area and was on Friday heading towards its original destination of Mombasa, Kenya, according to Kevin Speers, a spokesman for Maersk.

The ongoing standoff has also attracted the attention of the United States' top military and civilian officials.

President Barack Obama has been receiving regular updates and Defence Secretary Robert Gates on Thursday said he was watching the situation "very closely."

Other pirate-captured vessels were also apparently on their way to the scene in a show of solidarity, carrying guns and hostages taken from previous seizures, though the pirates were apparently unable to get close to the US fleet, media reported.

Phillips has been in contact with the US Navy and his own crew through a radio and has been given extra batteries and provisions, according to Speers.

Observers said the hostage drama could 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. The lifeboat has about 10 days' supplies of food and water, media reports said.

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

In 2008, pirates seized dozens of vessels in and around the Gulf of Aden and collected tens of millions of dollars in ransom, 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.

Observers have 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.