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

Meanwhile, a Belgian government spokesman said that a Belgian-registered ship with a 10-member crew, including two Belgians, was feared hijacked by Somali pirates.

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 navies off the east African coast.

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-managed tanker by pirates firing assault rifles and grenades.

The Dutch ship 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 said. The Corte-Real is also on a Nato mission.

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

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