Dutch sailors taking part in NATO's piracy-busting operation Ocean Shield on Thursday detained seven suspected pirates off the coast of Somalia after an attack on a Spanish vessel.

The alleged pirates were intercepted by Dutch amphibian transport HMS Rotterdam after the Izurdia was ambushed by a small boat off the Horn of Africa, the Dutch defence ministry said in a statement.

"The suspected skiff was stopped after warning shots were fired and a helicopter-borne team boarded the boat," the Rotterdam's commander Huub Hulsker said in the statement.

"The boarding team detained seven suspects who were taken on board the Rotterdam," he added.

Defence ministry spokesman Jos van der Leij told AFP the Izurdia managed to repel the attack, but did not give details.

The 12,700-tonne Rotterdam sailed to Aden on July 11 to take part in Operation Ocean Shield, the defence ministry added on its website.

Statistics from the European Union Naval Force's anti-piracy operation Atalanta say that after a spike last year successful pirate attacks on commercial vessels off the Horn of Africa have diminished.

In 2011 some 151 attacks were recorded, while only 29 took place so far this year, it said on its website.

Pirates currently control six ships and hold hostage an estimated 156 crew members. More than 1,000 alleged pirates are awaiting trial or have been sentenced in 20 countries.