MOMBASA, Kenya — Pirates vowed to hunt down American ships and kill their sailors Wednesday and French forces detained 11 other hijackers in a high-seas raid, raising tensions a day after an abortive attack on a U.S. freighter loaded with food aid.

Pirates fired grenades and automatic weapons at the Liberty Sun, but its American crew successfully blockaded themselves inside the engine room. The ship was damaged in Tuesday's attack but escaped and was heading to Kenya under U.S. navy guard.

A pirate whose gang attacked the aid ship admitted Wednesday that his group was targeting American ships and sailors.

"We will seek out the Americans and if we capture them we will slaughter them," said a 25-year-old pirate based in the Somali port of Harardhere who gave only his first name, Ismail.

"We will target their ships because we know their flags. Last night, an American-flagged ship escaped us by a whisker. We have showered them with rocket-propelled grenades," boasted Ismail, who did not take part in the attack on the Liberty Sun.

The threat came after U.S. navy sharpshooters killed three pirates Sunday to win the release of a hijacked American sea captain, Richard Phillips of the Maersk Alabama.

The French forces, meanwhile, launched an early morning attack on a pirate ship after spotting it Tuesday with a surveillance helicopter and observing the pirates overnight. The raid thwarted the bandits' planned attack on the Liberian cargo ship Safmarine Asia, the French Defence Ministry said.

The statement called the pirate vessel a mothership -- usually a seized foreign ship that pirates use to transport speedboats far out to sea and resupply them. The ship was intercepted about 900 kilometres east of the Kenyan city of Mombasa.

The 11 detained pirates were being held on the Nivose, a French frigate among the international fleet trying to protect shipping in the Gulf of Aden. France already is holding several pirates for prosecution.

Tuesday's attack on the Liberty Sun foiled the reunion between Phillips and the 19-man crew he saved with his heroism. Phillips had planned to meet his crew in Mombasa and fly home with them Wednesday, but was stuck on the USS Bainbridge when it was diverted to help the Liberty Sun.

The crew left without him, flying to Andrews Air Force base in Maryland in a chartered plane.

The Liberty Sun had left Houston with a crew of 20 American sailors and a load of aid for the World Food Program. It warded off the pirates with evasive manoeuvres, according to Lt. Nathan Christensen of the Bahrain-based U.S. 5th Fleet.

Despite President Barack Obama's vow to take action against the rise in banditry and the deaths of five pirates in French and U.S. hostage rescues, brigands have seized four vessels and more than 75 hostages since Sunday's dramatic rescue of Phillips.

Pirates released a Greek-owned cargo ship Wednesday and Greek authorities said all 24 crewmen on the Titan were in good health. The ship had been hijacked March 19 in the Gulf of Aden.

In all, Somali pirates are holding over 280 sailors on 15 ships -- at least 76 of those sailors captured in the past few days. Pirates have attacked 79 ships this year and hijacked 19 of them, according to the International Maritime Bureau, a piracy watchdog.

Pirates can extort $1 million US or more for each ship and crew seized off the Horn of Africa -- and Kenya estimates they raked in $150 million last year.

The United States has asked the International Committee of the Red Cross and officials in Somalia to help locate the families of the three pirates slain Sunday by navy snipers so their remains can be returned, a senior U.S. official said on condition of anonymity because he was not authorized to speak on the record.