Somali pirates have attacked another US merchant ship off the coast of Somalia using rocket-propelled grenades and automatic weapons.

The pirates damaged the Liberty Sun, which was carrying a cargo of food aid, but were not able to board it, reports the BBC.

After coming under fire, the Liberty Sun immediately requested assistance from the USS Bainbridge, said owners Liberty Maritime Corp in a statement.

The navy destroyer arrived some hours later, by which time the pirates had gone.

The BBC said that the latest attack shows the defiance and danger posed by pirates off the Somali coast.

"We are grateful and pleased that no-one was injured and the crew and the ship are safe," said the Liberty Maritime Corp statement.

The ship did sustain some damage, it said, but was able to resume its journey to Mombasa.

In the last 48 hours, four vessels have been seized in the same area. Gunmen in up to four skiffs took a Lebanese-owned cargo ship, the MV Sea Horse. A Greek-owned bulk carrier, the MV Irene was also seized. Two Egyptian fishing boats were held the previous day.

Meanwhile, three Somali pirates who had taken French hostages are in custody in France, French prosecutors say. The pirates were captured during a military operation to free hostages taken on the Tanit, a French yacht overtaken by hostages in the Gulf of Aden on April 4.