* Dutch free 20 Yemeni hostages, briefly hold seven pirates
* Hostages had been forced to sail pirate "mother ship"
* Belgian ship with 10 crew hijacked by sea gang
(Adds details/quotes from Belgian crisis centre)
By Mohamed Ahmed
MOGADISHU, April 18 (Reuters) - 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.
"The Pompei is heading slowly towards the Somali coast," Peter Mertens, a spokesman for a Belgian government crisis centre, said. "We have had visual contact from a helicopter of a Spanish navy ship."
Somali sea gangs have captured dozens of ships, taken hundreds of sailors prisoner and made off with tens of millions of dollars in ransoms despite an unprecedented deployment by foreign navies in waters off the Horn of Africa.
The attacks have disrupted U.N. aid supplies, driven up insurance costs and forced some shipping companies to route cargo round South Africa, rather than risk approaching Somalia.
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, pursued 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.
Mertens said fears grew for the Pompei, a dredging vessel, after it sounded two alarms early on Saturday when it was about 600 km (370 miles) from the Somali coast en route to the Seychelles.
The ship was carrying two Belgian, four Croatian, one Dutch and three Filipino crew members.
A pirate source who said he was on board the Pompei told Reuters in Mogadishu by satellite phone that the pirates would sail it to a coastal base. "We have hijacked a Belgian ship. We will take it to Haradheere," he said.
Mertens said the Spanish Navy ship that sent the helicopter would probably make visual contact with the "Pompei" around 2000 GMT on Saturday, but did not have any special forces on board that could recapture the Belgian ship.
"We have to think about their (the crew's) security before doing anything stupid and moving too fast," he said. The pirates had not made any ransom demands.
Regional analysts and security experts say that without political stability in Somalia, which has been mired in conflict for 18 years, the pirate gangs will continue to thrive.
On Friday, five gunmen in a skiff approached a Danish cargo vessel, the MV Puma, in the Gulf of Aden, prompting U.S. and South Korean warships to send aircraft to the scene.
Last week pirates from the lawless Horn of Africa state captured two more ships and fired on two others. A French naval frigate seized 11 gunmen on Wednesday, foiling another attack.
The Somali government plans to present its proposals to combat the sea gangs at a major donors' meeting on Somalia in Brussels on Wednesday and Thursday.
It says it needs more money to tackle insecurity on land and to provide jobs for the country's many out-of-work young men.
Most of Somalia's pirate gangs operate from the semi-autonomous northern region of Puntland, where many of them say they first took to the seas to stop illegal fishing by European fleets and the dumping of toxic waste.
In a Reuters interview late on Friday, Puntland President Abdirahman Mohamed Farole also blamed ship owners for paying ransoms that encouraged impoverished youths to join the gunmen.
"But the root cause of this piracy, as everyone knows, is illegal fishing," Farole said in neighbouring Kenya.
"That situation still exists, so any activity directed at eliminating piracy should also be combined with the elimination of illegal fishing by foreign trawlers." - For complete coverage, click [ID:nPIRATES] (Additional reporting by Alison Bevege on board the Corte-Real, Abdi Guled in Mogadishu and Abdiaziz Hassan in Nairobi; Writing by Daniel Wallis; editing by Robert Woodward)