The US skipper freed in a dramatic rescue after being held hostage off Somalia for days arrived in Kenya on Thursday aboard a warship, while Washington said it wants to freeze pirates' assets.

Captain Richard Phillips arrived in the port of Mombasa aboard the USS Bainbridge, the warship behind his rescue last weekend, as the 19 American crew members from his ship, the Maersk Alabama, separately returned home.

Mombasa port police commander Ayub Gitonga confirmed Phillips was on the Bainbridge along with a fourth pirate, who survived the rescue operation in which three of his fellow bandits were killed by snipers.

Asked whether the surviving pirate would be tried in Kenya, Gitonga said: "The decision is yet to be made."

Crew members from the Danish-operated Maersk Alabama were flown to an air base outside Washington in the early morning hours after being in Mombasa since the vessel docked there on Saturday.

Pirates had attacked their ship in the Indian Ocean on April 8 and took Phillips hostage on a lifeboat after his crew managed to overpower the bandits.

After the US navy operation that rescued Phillips, pirates pledged to target Americans in revenge for the sniper killings.

On Tuesday, pirates said they attacked an American freighter with rockets to "destroy" the ship as revenge, but the Bainbridge came to the rescue of the freighter, the Liberty Sun, which escaped.

Aided by good weather, Somali pirates have intensified attacks off the lawless country's coast in recent days, with at least 10 ships seized since the beginning of this month.

The pirates have defied an international naval presence in the region to carry out the hijackings, which have wreaked havoc on one of the world's busiest shipping routes and threatened vital aid deliveries to African nations.

US Secretary of State Hillary Clinton unveiled a plan on Wednesday, calling for prosecution and freezing pirates' assets with the support of Washington's international partners.

The chief US diplomat added that she was also sending an envoy to an 23 April Somali donors' conference in Brussels to improve the situation in lawless Somalia and help implement the plan.

"These pirates are criminals, they are armed gangs on the sea," Clinton told reporters.

"And those plotting attacks must be stopped, and those who have carried them out must be brought to justice."

Clinton dismissed suggestions that it would be difficult to track the ill-gotten gains of pirates operating out of Somalia, which has seen nearly two decades of conflict and chaos, making piracy one of the few viable businesses.

"We track and freeze and try to disrupt the assets of many stateless groups," including Islamist extremists, Clinton said.

She said there are "ways to crack down on companies that do business with pirates."

The World Food Programme has warned that millions in Africa risk going hungry if pirate hijackings keep aid ships from arriving in Mombasa.

While piracy off Somalia has long been a problem for aid freighters, recent hijackings have marked a new development in the attacks, it said in a statement.

The attack on the Maersk Alabama "was the first case of a Mombasa-bound ship carrying WFP food being hijacked," the agency said.

"If food assistance cannot arrive through Mombasa for Somalia, Kenya, Uganda, southern Sudan and the eastern Democratic Republic of Congo, millions of people will go hungry and the already high malnutrition rates will rise."

Sources close to the pirates have said French ships are now also a prime target after French commandos recently stormed a yacht on which two French couples and a child were held. One male hostage and two pirates were killed.

The French navy on Wednesday also intercepted a pirate "mother ship" in the Gulf of Aden and detained 11 fighters, the French defence ministry said.