The US and its allies battled Somalia's pirates on two fronts Wednesday, with French forces seizing a bandit mother ship and Washington seeking to keep the marauders from their spoils. Another US freighter headed to port with armed sailors aboard after pirates damaged it with gunshots and grenades.

One pirate issued a new threat to "slaughter" Americans, and Tuesday's assault on a second US cargo ship, the Liberty Sun, underscored the outlaws' ability to act with impunity despite international naval operations against them and mounting concern worldwide over how to end the escalating attacks off the Horn of Africa.

Pirates bombarded the US-flagged Liberty Sun with automatic weapons fire and rocket-propelled grenades, but its American crew of about 20 successfully blockaded themselves in the engine room and warded off the attack with evasive maneuvers.

The ship, carrying food aid for hungry Africans - including Somalis - was damaged "pretty badly" on its bridge, a US official said on condition of anonymity because he was not authorized to speak on the record about the ship.

Windows were blown out and the crew had to put out a small fire, the official said, but they were still able to navigate. By the time the USS Bainbridge arrived five hours later, the pirates were gone.

Meanwhile, French naval forces launched an early-morning attack on a suspected pirate "mother ship" 550 miles east of Mombasa and seized 11 men, thwarting an attack on the Liberian cargo ship Safmarine Asia, the French Defense Ministry said. No one was injured.

The ministry said the vessel was a larger ship that pirates use to allow their tiny skiffs to operate hundreds of miles off the coast.

French Foreign Ministry spokesman Christophe Prazuck said a French helicopter in the area heard a distress call from the Safmarine Asia. He described the seized ship as a small, noncommercial vessel carrying fuel, water and food supplies.

The 11 pirates, believed to be Somalis, were being held on the Nivose, a French frigate among the international fleet trying to protect shipping in the Gulf of Aden.

France has been proactive against pirates for at least the past year, intervening to save three of its ships and spearheading a Europe-wide anti-piracy force called Atalanta. French politicians have sought to have other European countries take greater action against pirates.

Three Somali pirates in the French city of Rennes faced judicial investigation after being captured in a hostage rescue Friday. Several other pirates also have been in French custody since last year.

In Washington, Secretary of State Hillary Rodham Clinton announced new diplomatic efforts to freeze the pirates' assets and said the Obama administration will work with shippers and insurers to improve their defenses against pirates, part of a diplomatic initiative to thwart attacks on shipping.

"These pirates are criminals, they are armed gangs on the sea. And those plotting attacks must be stopped," Clinton said at the State Department.

Clinton did not call for military force, although she mentioned "going after" pirate bases in Somalia, as authorized by the UN several months ago.

She said it may be possible to stop boat-building companies from doing business with the pirates.

The measures outlined by Clinton are largely stopgap moves while the administration weighs more comprehensive diplomatic and military action.

She acknowledged it will be hard to find the pirates' assets. But she wants the US and others to "explore ways to track and freeze" pirate ransom money and other funds used in purchases of new boats, weapons and communications equipment.