Somali pirates have captured a US tug and its 16-strong crew, a Kenyan official reported on Saturday, hours after another group holding a US captain hostage warned against any attempt to free him.

French Defence Minister Herve Morin meanwhile defended Friday's marine raid on a French yacht in the region that left one hostage and two pirates dead.

Somali pirates captured the US-owned, Italian-flagged tug and its 16-strong crew on Saturday at around 11am local time, Andrew Mwangura of Kenya's East African Seafarers Assistance Program told AFP.

"The 16 crew members are all safe," he added. But he did not know the name of the boat.

Ten of the 16 crew are Italians, Italy's foreign ministry said in a statement, without giving any details.

It was just the latest in a series of raids in the Gulf of Aden and the Indian Ocean, despite the presence of an international task force in the region to defend international shipping using the busy passage.

Meanwhile, pirate commander Abdi Garad, speaking from the northern Somali pirate lair of Eyl, told AFP that US Captain Richard Phillips would be moved from the lifeboat where he was being held to another ship off the Somali coast.

On Friday, Phillips jumped into the water and tried to swim towards the nearby US destroyer the USS Bainbridge, but was recaptured.

Only four pirates are guarding him on the lifeboat, but transfer to a larger ship would make it easier for them to thwart any US military attempt to free him.

US Navy forces have been pouring into the region amid the standoff over Philipps, who has been held hostage since Wednesday, since the Danish-operated container ship he commanded was attacked.

The four pirates hijacked the Maersk Alabama, carrying 5,000 tonnes of UN aid destined for African refugees.

Its unarmed American crew managed to take back control of the ship, but the pirates bundled Phillips onto the lifeboat as they escaped.

Any attempt to free him would be disastrous, warned Garad.

"I'm afraid this matter is likely to create disaster because it's taking too long and we are getting information that the Americans are planning rescue tricks like the French commandos did," he said.

Somali elders and parents of pirates holding the American were trying to free the American on Saturday "without any guns or ransom", said Mwangura in a statement from Kenya.

But a spokesman for Somalia's hardline Islamist group Shebab backed the recent rash of pirate attacks.

"I believe that the pirates are not wrong to hijack the ships because there is no reason for the international ships to use Somalia's waters," Sheikh Muktar Robow told reporters in the south-central Somali city of Baidoa.

He blamed the international community for depleting the country's marine resources and dumping their waste there.

Officers of the US Federal Bureau of Investigation meanwhile were waiting to debrief the crew of the Maersk Alabama on their arrival at the Kenyan port of Mombasa, said a statement from the ship's owners Maersk.

Friday's raid by French marines came six days after the yacht, the Tanit, was seized in the Gulf of Aden.

Although French forces freed three adults and a three-year-old boy, a fourth man, Florent Lemacon, the owner of the yacht and the child's father, was killed.

French Defence Minister Herve Morin said on Saturday an autopsy and investigation would determine what had happened. But he could not rule out that the fatal shot had come from the French forces.

"We did everything to save the hostages' lives," Morin said on French radio.

The Tanit had been heading toward the coast and the pirates' threats were "becoming more and more specific", said a spokesman from the office of French President Nicolas Sarkozy on Friday.

France's policy is not to let pirates take their hostages ashore, he added.

The four ex-hostages - Lemacon's wife Chloe, their three-year-old son Colin and two other adults - were due in Paris on Sunday aboard a French-chartered plane, Morin told AFP.

Sarkozy had spoken by telephone with both Chloe Lemacon and the dead man's father on Saturday, said a spokesman from his office.