Dutch commandos freed 20 Yemeni hostages on Saturday and briefly detained seven pirates who had forced the Yemenis to sail a "mother ship" attacking vessels in the Gulf of Aden, Nato officials said.

In a separate incident, gunmen from Somalia seized a Belgian-registered ship and its 10 crew, including seven Europeans, further south in the Indian Ocean. A pirate source said the vessel, the Pompei, would be taken to the coast.

Somali sea gangs have captured dozens of ships, taken hundreds of sailors prisoner and made off with millions of dollars in ransoms despite an unprecedented deployment by foreign warships in waters off the Horn of Africa.

Nato Lieutenant Commander Alexandre Fernandes, speaking on board the Portuguese warship Corte-Real, said the 20 fishermen were rescued after a Dutch navy frigate on a Nato patrol responded to an assault on a Greek-owned tanker by pirates firing assault rifles and grenades.

Commandos from the Dutch ship, the De Zeven Provincien, chased the pirates, who were on a small skiff, back to their "mother ship" - a hijacked Yemeni fishing dhow.

"We have freed the hostages, we have freed the dhow and we have seized the weapons... The pirates did not fight and no gunfire was exchanged," Fernandes told Reuters. The Corte-Real is also on a NATO anti-piracy mission.

He said the hostages had been held since last week. The commandos briefly detained and questioned the seven gunmen, he told Reuters, but had no legal power to arrest them.

"Nato does not have a detainment policy. The warship must follow its national law," he said.

"They can only arrest them if the pirates are from the Netherlands, the victims are from the Netherlands, or if they are in Netherlands waters."

He said an unexploded rocket-propelled grenade was later found on board the tanker, the Marshall Islands-flagged MT Handytankers Magic. managed by Roxana Shipping SA of Greece.

A Belgian government crisis center spokesman said fears grew for the Pompei, a dredging vessel, after it put out two alarm signals early on Saturday when it was about 600 km from the Somali coast en route to the Seychelles.

Fernandes said the ship was carrying two Belgian, four Croatian, one Danish and three Filipino crew members.

"A helicopter from EU naval force Operation Atalanta flew over and confirmed the hijacking visually," he told Reuters.

A pirate source who said he was on board the Pompei told Reuters in Mogadishu by satellite telephone that the pirates would take it to a coastal base. "We have hijacked a Belgian ship. We will take it to Haradheere," he added.-Reuters