NATO attacked a pirate "mothership" on Saturday and freed 20 captured fishermen, but were forced to let a gang of Somali hijackers go free during a day of high drama off the east coast of Africa.

NATO forces were unable to detain the seven captured pirates because they lacked jurisdiction in the area to arrest them.

Hours earlier in a separate attack, bandits captured a Belgian-flagged ship carrying 10 foreign crew members as it sailed south of the Horn of Africa.

Both incidents illustrate the ongoing difficulty of effectively policing and maintaining safety off of Africa's east coast, where brash bands of pirates have carried out an increasing number of effective attacks in recent months.

This year, pirates have launched attacks on more than 80 vessels and now hold at least 18 ships and over 310 crew members hostage, according to The Associated Press.

In 2003, about 20 ships were attacked.

Pirates hijacked the Belgian-flagged Pompei in the Indian Ocean, about 240 kilometres north of the Seychelles, according to Portuguese Lt.-Capt. Alexandre Santos Fernandes, a member of the NATO team patrolling the area.

The ship sent two warning signals early Saturday morning that it was being attacked.

Officials have not been able to make contact with the ship since the signals were sent out.

"There is no contact with the pirates, not with the crew, not with any other parties," said Jaak Raes, director general of the Belgian Crisis Center. "We are sure that the ship now is heading to the coast of Somalia."

The Pompei has a crew of 10: two Belgians, one Dutch, three Filipinos and four Croatians.

Hours later, the Marshall Islands-flagged Handytankers Magic sent out a distress call after it was attacked by pirates armed with guns and rockets, Fernandes said.

A Dutch frigate that is part of the NATO force saw the pirates attempting to escape "on a small white skiff, which tried to evade and proceed toward a Yemeni-flagged fishing dhow," that the same pirates had seized on Sunday, Fernandes said.

The pirates were using the Yemeni boat as a so-called "mother ship," from which to launch skiffs to conduct attacks far away from the Somali coast.

The Dutch ship intercepted the skiff and captured the seven pirates, in addition to seizing seven Kalashnikov rifles and a rocket-propelled grenade launcher.

The NATO troops then boarded the Yemeni vessel, freeing the 20 fishermen that had been held hostage on board.

A number of ships from Europe and North America, including HMCS Winnipeg, are battling pirates in the Gulf of Aden, one of the world's busiest shipping lanes thanks to its status as a short cut between Europe and Asia.

Pirates have netted millions of dollars in recent years attacking ships and holding crew members for ransom.

With files from The Associated Press