Nairobi/The Hague  - A Dutch naval unit Saturday freed 16 Yemeni fishermen and their vessel after boarding a boat used by Somali pirates to attack a tanker in the Gulf of Aden, a Dutch military spokesman said in The Hague.

He said nine pirates were briefly detained but then released after their weapons were seized. Pirates had been in a speedboat operating from the fishermen's dhow which they had been using as a "mother ship" after seizing it earlier in the week.

They had first been fired upon by a British naval vessel, and were then successfully pursued by the Dutch frigate Zeven Provincien to the "mother ship" in an operation coordinated by NATO naval forces.

There, the Dutch crew discovered 25 people on board, including the 16 fishermen, who were freed and were able to sail off in the repossessed dhow.

The dhow was one of four vessels seized within 48 hours at the beginning of the week, and had been used by the pirates to launch attacks on other ships.

The Dutch warship had been responding to a distress call from a Greek-operated tanker that came under attack early Saturday.

However, the Dutch military spokesman said the pirates had been released after their weapons - seven AK-47 rifles and a rocket- launcher - were seized and destroyed.

Dutch law did not allow them to be held under the circumstances they were captured in, the spokesman said, as they had not offered resistance and had not been involved in any attack on the Dutch crew.

Meanwhile, a Belgian dredging vessel was also believed to have fallen into pirate hands.

The Belgian government said that the Pompeii sent two distress calls early on Saturday. Attempts to communicate with the vessel have since failed. The ship's ten crew members include Belgians.

Pirate activity has picked up in recent weeks after a lull due to bad weather. Almost 30 ships have been attacked in the last four weeks. Sixteen ships, with around 250 crew members, are being held hostage.

The highest-profile attack in recent weeks was the failed hijacking of the US-flagged Maersk Alabama.

The Alabama's crew repelled the hijacking but the captain was held hostage for five days on a lifeboat. He was freed Sunday when US Navy snipers killed the three pirates holding him.

Pirates have vowed to take revenge on citizens of US and France, whose naval forces recently killed two pirates during an operation to free a French yacht. The captain of the yacht also died in the attack.

In 2008, pirates seized more than 40 vessels in and around the Gulf of Aden and collected tens of millions of dollars in ransoms, prompting the international community to send warships to the region.

According to the US Navy, around 15 warships from the European Union, a US-led coalition task force, NATO and individual countries patrol an area of about 2.85 million square kilometres.

The presence of the warships has appeared to have had little effect. Observers say that only by tackling insecurity and poverty in Somalia will piracy finally be halted.

Somalia has been without a functioning government since the 1991 ouster of dictator Mohamed Siad Barre.

US Secretary of State Hillary Rodham Clinton on Wednesday announced a four-point plan to counter piracy, including steps to improve security, policing and governance in Somalia. (dpa)