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A U.S.-flagged ship freed from Somali pirates has reached its destination, but the ship's captain is still being held hostage on a lifeboat in the Indian Ocean.

The Maersk Alabama and its 19 remaining crew members arrived in Kenya's Mombasa harbour Saturday, where it will deliver food aid. Its crew had been able to overpower the pirates during a failed hijacking last week.

But the pirates are still holding Capt. Richard Phillips on a small vessel. U.S. Navy ships are trying to prevent them from receiving reinforcements, in what has become a tense four-day standoff.

According to a diplomat based in Nairobi, who spoke to the Associated Press on condition of anonymity, pirates holding Phillips have appealed for help.

However, two U.S. ships, as well as surveillance aircraft from the U.S. Navy, are preventing pirate ships and skiffs from contacting the lifeboat.

A Somali who has communicated with the pirates said the captors have asked for help from four commandeered ships from countries including the Philippines, Russia and Germany.

The captors' strategy is to get Phillips to Somalia, hide him to ward off a rescue operation and begin negotiations for a ransom, the Somali said.

He also said the pirates have indicated they will kill Phillips if they come under attack by the U.S. military.

The Philippines has the most sailors currently being held captive by Somali pirates.

Vice President Noli de Castro called for the United States and other countries to ensure the safety of all hostages during the ongoing standoff.

"We hope that before launching any tactical action against the pirates, the welfare of every hostage is guaranteed and ensured," de Castro said Saturday. "Moreover, any military action is best done in consultation with the United Nations to gain the support and cooperation of other countries."

The high-seas drama began on Wednesday, when pirates attempted to take over the Maersk Alabama, which was loaded with food aid for people in Somalia, Rwanda and Uganda.

The pirates held the ship for several hours but were overpowered by about 20 crew members.

When the pirates fled the 17,000-ton cargo ship in a lifeboat, they took Phillips with them as a hostage.

The lifeboat has since run out of fuel and is now surrounded by U.S. Navy ships, including the USS Bainbridge, which is equipped with rescue helicopters and lifeboats, and the USS Hallyburton, which also has helicopters.

The USS Boxer is expected to arrive on the scene soon. The ship is part of a multination anti-piracy task force, has a crew of more than 1,000, a mobile hospital, missile launchers and about two dozen helicopters and attack planes.

However, U.S. rules of engagement prevent the Americans from launching an offensive against the pirates if such a move would endanger civilians.

On Friday, the tense standoff almost came to an end when Phillips managed briefly to escape his captors by jumping out of the lifeboat and swimming away. However, he was quickly recaptured after one of the pirates fired a gun at or near him.

U.S. defence officials said that negotiations between the pirates and the captain of the Bainbridge are ongoing.

In another development Saturday, the head of the East African Seafarers' Assistance Program said that Somali pirates had hijacked an American-owned tugboat in the Gulf of Aden.

Andrew Mwangura said his organization had been informed that the U.S. tugboat, which flies an Italian flag, was towing two barges when it came under attack.

It is unclear if the attack, which occurred around 11 a.m. local time, took place off the coast of Somalia or further north near Yemen, Mwangura said.

The Italian Ambassador to Kenya, Pierandrea Magistrati could only confirm that "there is a boat that has been hijacked, I believe by Somali pirates."

Canada fighting pirates

In yet another dramatic moment in the Gulf of Aden, the commander of a Canadian navy ship says his crew helped prevent a suspected pirate attack on a civilian vessel late Friday night.

Commander Craig Baines of HMCS Winnipeg said he sent out the frigate's Sea King helicopter after the civilian ship reported being fired upon by four people in a skiff.

When Baines spotted the vessel firing the shots, he sent a boarding team over to the skiff to conduct a search for weapons.

"As we approached the vessel it was the dark of night and they actually were throwing items overboard, which we believe were possibly the weapons used in the attack," Baines told Â鶹ӰÊÓnet on Saturday.

The Winnipeg is currently serving as part of a NATO-led anti-piracy mission called Operation Allied Protector.

Somali pirates have been engaged in what Baines called "armed robbery at sea," as they seize ships and anchor them near the shore.

The pirates secure their hostages on the ship before escaping to shore to negotiate for millions of dollars in ransom.

It is estimated that pirates currently hold about a dozen ships with more than 200 crew members, according to the International Maritime Bureau, a piracy watchdog group.

With files from The Associated Press