Nairobi - Another US ship carrying humanitarian aid came under attack by Somali pirates on Tuesday.
Damage was caused but the crew were not injured, according to the owner of the ship.
The Liberty Sun, with its crew of about 20 Americans, was on its way to Mombasa, Kenya, when the pirates fired rocket-propelled grenades and automatic weapons at it, the ship's owner Liberty Maritime Corp. said.
The cargo ship carried food for famine-stricken nations in Africa.
US Navy sailed to aid when the vessel reported being attacked but the pirates had left when the destroyer Bainbrighe arrived some six hours later, US Central Command spokesman Jack Hanzlik said.
The vessel continued on its way to Kenya under Navy escort although a rocket penetrated the bulkhead and caused a small hole, the company said.
President Barack Obama vowed Monday to "halt the rise of piracy," as Somali pirates continue attacking and hijacking ships on the open sea. They seized a total of four ships with 60 hostages on Tuesday.
On Sunday night, the US cargo ship captain taken captive by Somali pirates was freed without injury after days of ordeal.
US soldiers aboard warships launched a raid on the lifeboat in which the four pirates held US cargo ship captain Richard Phillips, killing three hijackers and taking one in custody.
At least three US warships including the destroyer USS Bainbridge were present when the navy operation began.
The captured pirate could face life imprisonment as piracy and hostage-taking carry life sentences under US law.