The US skipper freed in a dramatic rescue after being held hostage off Somalia for days arrived in Kenya Thursday aboard a warship, while Washington said it wants to freeze pirates' assets. [...] 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 April 23 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. [...] 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."