French and US warships on patrol off the Somali coast combined to detain 12 suspected pirates after a merchant vessel reported coming under attack at the weekend, a statement said Tuesday.

NATO and the EU's Atalanta anti-piracy taskforce said the vessel issued a distress call after it was attacked by a boat carrying six suspected pirates armed with rocket-propelled grenades.

NATO said the crew on the unnamed Panama-flagged vessel gathered in a secure room while an on-board Armed Security Team exchanged fire with the suspected pirates.

The USS Halyburton and the French frigate Surcouf responded to the distress call, with French forces then intercepting two small boats with 12 men on board in all.

The 12 are now being held on the Surcouf, pending legal proceedings, a statement said.

Operation Atalanta is part of an array of anti-piracy forces in the area which has been plagued by pirate attacks on shipping for years, although the number of incidents has fallen steadily in the past year.