NAIROBI, Kenya - Somali pirates attacked a tanker in the Gulf of Aden on Saturday, but NATO forces chased them down and freed 20 Yemeni fisherman the pirates were holding on another hijacked boat.

A second ship was also reported to be under attack by pirates in the Indian Ocean, further southeast near the Seychelles islands.

The latest violence underscores the dangers in the seas off the Horn of Africa nation. Pirates from anarchic, clan-ruled Somalia have attacked more than 80 boats this year and still hold 15 cargo ships and more than 280 crew members hostage.

The Marshall Islands-flagged Handytankers Magic issued a distress call shortly after dawn when pirates attacked it with small arms and rockets in the Gulf of Aden, said Portuguese Lt.-Capt. Alexandre Santos Fernandes, who is traveling with the NATO fleet patrolling the region.

A Dutch frigate from the NATO force spotted the pirates fleeing "on a small white skiff, which tried to evade and proceed toward a Yemeni-flagged fishing dhow" that had been seized by the pirates Sunday, Fernandes said.

He said pirates were using the Yemeni vessel as a "mother ship," a boat that allows the pirates' tiny skiffs to operate far off the Somali coast.

The Dutch ship intercepted the pirate skiff and its forces then boarded the Yemeni vessel, freeing 20 Yemeni fishermen. They also detained seven pirates and seized seven Kalashnikov rifles and one rocket-propelled grenade launcher.