Somali pirates hijacked an Italian-flagged tugboat with 16 crew Saturday, a NATO spokeswoman said, as U.S. warships closely watched a lifeboat where an American captain was being held hostage for a fourth day.

The tugboat was hijacked in the Gulf of Aden off Somalia's northern coast as it was pulling barges, said Shona Lowe, a spokeswoman at NATO's Northwood maritime command center.

The Foreign Ministry in Rome confirmed 10 of the 16 crew members are Italian. Italian government officials and the company that owned the vessel were involved in trying to secure their release, Lowe said.

The ship is owned by an Italian maritime services company, Micoperi. It was traveling from Singapore to transfer two empty barges to the Port of Suez, Fabio Bartolotti, the son of the company's owner, told The Associated Press.

The attack on the Italian boat took place as the American captain of the U.S.-flagged Maersk Alabama was held on a lifeboat watched by two U.S. warships, hundreds of miles from land.

The Alabama was heading toward the Kenyan port of Mombasa — its original destination — with 20 American crew members aboard. It was expected to arrive Saturday night, said Joseph Murphy, whose son is second-in-command of the vessel.

Port officials moved shipping containers Saturday afternoon to block reporters' and photographers' views of the ship when it docks.

A Nairobi-based diplomat, who receives regular briefings on the situation, said the four pirates holding Capt. Richard Phillips some 380 miles off shore had tried to summon other pirates from the Somali mainland.

The diplomat, who spoke on condition on anonymity because he was not authorized to talk to reporters, said that pirates had been trying to reach the lifeboat.

He said that at least two American ships and U.S. Navy surveillance aircraft had been attempting to deter pirate ships and skiffs from contact with the lifeboat but he did not know if the pirates and Navy ships had come into contact.

A Somali who described himself as having close ties to pirate networks told The Associated Press that pirates had set out in four commandeered ships with hostages from a variety of nations including the Philippines, Russia and Germany. The diplomat told the AP that large pirate "motherships" and skiffs were heading in the direction of the lifeboat.

A second Somali man who said he had spoken by satellite phone to a pirate piloting a seized German freighter told the AP by phone Saturday that the pirate captain had reported being blocked by U.S. forces and was returning Saturday to the pirate stronghold of Harardhere.

Speaking on condition of anonymity for fear of reprisals, the Somali man said the pirate captain told him the ship was in sight of a U.S. Navy destroyer Saturday morning local time, received a U.S. warning not to come any closer and, fearing attack, left the scene without ever seeing the lifeboat.

A Pentagon official who spoke on condition of anonymity because of the sensitivity of the negotiations said in Washington Saturday morning that there had been no developments overnight. He declined to comment on the report that the U.S. Navy had turned back the pirates.

The Somali man said the pirate also told him that two other commandeered ships from Taiwan and Greece that were trying to reach the lifeboat feared a showdown with the U.S. Navy and returned to Eyl, a port that serves as a pirate hub, on Friday night. It was not immediately possible to contact people in Eyl Saturday.

The Somali man said the fourth ship that had tried to reach the lifeboat was a Norwegian tanker that was released Friday after a $2 million ransom was paid. The owner of the Norwegian tanker Bow Asir confirmed Friday that it had been released two weeks after it was seized by armed pirates off the Somali coast, and all 27 of its crew members were unhurt.

Phillips, 53, of Underhill, Vermont, was seized Wednesday when he thwarted the takeover of the 17,000-ton U.S.-flagged Maersk Alabama, which was carrying food aid for hungry people in Somalia, Rwanda and Uganda. He told his crew of 20 to lock themselves in a cabin, crew members told stateside relatives.

Phillips surrendered himself to safeguard his men. The crew later overpowered some of the pirates but the Somalis fled with the captain to an enclosed lifeboat, the relatives said.

On Friday, Phillips jumped out of the lifeboat and tried to swim for his freedom but was recaptured when a pirate fired an automatic weapon at or near him, according to U.S. Defense Department officials speaking on condition of anonymity because they are not authorized to talk about the sensitive, unfolding operations.

Negotiations had been taking place between the pirates and the captain of the Bainbridge, who was getting direction from FBI hostage negotiators, the officials said.

Sailors on the USS Bainbridge, which has rescue helicopters and lifeboats, were able to see Phillips but at several hundred yards away were too far to help him. The U.S. destroyer is keeping its distance, in part to stay out of the pirates' range of fire.

The lifeboat has some gas and the ability to move, according to U.S. defense officials who spoke on condition of anonymity in order to discuss sensitive details.

U.S. sailors saw Phillips moving around and talking after his return to the lifeboat, and the Defense Department officials said they think he is unharmed.

The Bainbridge was joined Friday by the USS Halyburton, which has helicopters, and the huge, amphibious USS Boxer was expected soon after, the defense officials said. The Boxer, the flagship of a multination anti-piracy task force, resembles a small aircraft carrier. It has a crew of more than 1,000, a mobile hospital, missile launchers and about two dozen helicopters and attack planes.

The vice president of the Philippines, the nation with the largest number of sailors held captive by Somali pirates, appealed for the safety of hostages to be ensured in the standoff.

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

On Friday, the French navy freed a sailboat seized off Somalia last week by other pirates, but one of the hostages was killed.

France's defense minister promised an autopsy and investigation into the death of the hostage killed during the commando operation, which freed four other captives and was prompted by threats the passengers would be executed.

The pirates had seized the sailboat carrying Florent Lemacon, his wife, 3-year-old son and two friends off the Somali coast a week ago.

Two pirates were killed, and Lemacon died in an exchange of fire as he tried to duck down the hatch. Three pirates were taken prisoner in the operation, and are to be brought to France for criminal proceedings.

Piracy along the anarchic and impoverished Somali coast, the longest in Africa, has risen in recent years.

Somali pirates have been seizing ships with many hostages and anchoring it near shore, where they have quickly escaped to land and begun negotiations for multimillion-dollar ransoms.

They hold about a dozen ships with more than 200 crew members, according to the International Maritime Bureau, a piracy watchdog group based in Malaysia. The bureau lists 66 attacks since January, not including the Alabama.