"This morning we intercepted a request for assistance from … the Handytankers Magic, that had fallen victim to a pirate attack," Lieutenant Commander Alexandre Fernandes told AFP from aboard the Portuguese frigate Corte Real, which is under the NATO flag.

"We immediately dispatched a Dutch NATO ship."

A Dutch defence ministry spokesman said the attack on the tanker had been launched from a dhow, a traditional Arab sail boat, which was itself captured by pirates last Thursday.

As the pirates sought to flee in small boats after the failed attack, a British naval vessel in the vicinity kept its guns trained on the group until the Dutch frigate arrived under NATO orders to board the dhow.

The frigate, part of a NATO anti-piracy patrol operation, Allied Protector, had been escorting four merchant vessels some 10 miles away.

"The marines found 25 people on board, nine of them suspected pirates," spokesman Robin Middel told AFP.

They also found seven AK47 assault rifles and a rocket launcher, which were seized and destroyed.

"The other people on the dhow were Yemeni fishermen who were hijacked by the suspected pirates."

Middel said the suspects had to be freed on the instructions of a NATO squadron commander.

"There exists no legal framework in the NATO for arrests to be carried out," he explained.

Belgium meanwhile confirmed a ship which had sent distress signals early Saturday had been captured.

Benoit Ramacker, a spokesman for the Belgian government’s crisis management centre, told AFP the 65-metre (213-foot) Pompei was spotted after having transmitting two alarms in the early morning hours and contact was lost.

"The ship has been spotted and we can confirm that it has been taken hostage," he said.

The double alert is a standard practice linked to anti-terrorism measures, according to the crisis centre.

The 1,850-tonne ship had a crew of two Belgians and a number of other nationalities, which earlier reports said included Dutch, Croatian and Filipino sailors.

Somali pirates attacked more than 130 merchant ships in the Gulf of Aden last year, an increase of more than 200 percent on 2007, according to the International Maritime Bureau which tracks piracy and shipping security issues.

Heavily armed pirates operate high-powered speed boats and sometimes hold ships for weeks before releasing them for large ransoms paid by governments or ship owners.

Dutch authorities already have five Somali pirates in custody awaiting trial.

The men were intercepted in the Gulf of Aden as they allegedly attacked a Dutch cargo ship in January. They risk up to nine years in jail if found guilty, and their leader up to 12 years.

More than 150 suspected pirates were arrested by naval patrols in the Gulf in 2008.

Ramacker said senior Belgian government and shipping officials had convened an emergency meeting in Brussels following the latest hijacking.

AFP/Expatica