Pirates have seized a Belgian ship in the waters off east Africa, a spokesman for the Belgian government's crisis management centre has said.

The 65-metre Pompei had transmitted an alarm at 04:30 GMT, followed by another half-an-hour later on Friday.

Benoit Ramacker, of the crisis management centre, said: "The ship has been spotted and we can confirm that it has been taken hostage."

He did not give further details, but said the double alert made by the ship is a standard practice linked to anti-terrorism measures, according to the crisis centre.

Peter Martens, also from the crisis management centre, said the Belgian authorities were trying to establish contact with the ship through diplomatic and military contacts and that they would "find out more in the coming hours hopefully".

Alexandre Fernandez, a Nato conmmander travelling with a fleet patrolling the Gulf of Aden, told Al Jazeera that the ship was hijacked at night.

"We know that the ship has 10 crew members - two Belgians, one Dutch, three Filipinos and four Croatians," he said.

The 1,850-tonne Pompei, belonging to the Jan de Nul group, has accommodation for 19 people.

It is used for installation of rock protection on offshore pipelines or for underwater rockberm construction, according to the company website.

Earlier, the Dutch navy embarked on a rescue operation that freed 20 Yemenis who had been held for more than a week.

Seven pirates, who surrendered without exchanging fire, were captured but were later freed by the navy for legal reasons.

Fernandes said that Nato did not have jurisdiction to prosecute pirates.