Somalian pirates have threatened to kill three crew members taken hostage from a Spanish fishing vessel if two suspected pirates held in Spain are not freed, the captain of the captured vessel said Friday.

Capt. Ricardo Blach told Spanish television by phone from the tuna boat Alakrana that the pirates have given a deadline of two days before they kill three of the ship's 33 crew members.

The two Somalians were captured shortly after the pirates overran the tuna boat on Oct. 2 and face charges of terrorism and robbery.

Spain's deputy defence minister Constantino Mendez said his country has ruled out releasing the two detained Somalian pirates as demanded, telling Spanish National Radio, "the situation is not negotiable."

But he did leave open the possibility the two men might be transferred to the court system of another country.

In May, Spanish naval forces caught seven young pirates attempting to hijack a Panamanian-flagged ship in the Gulf of Aden, but turned the men over to Kenya as part of an anti-piracy agreement with the European Union.

Somalia, which has not had an effective central government for 18 years, has became the world's piracy hotspot, and the Gulf of Aden, one of the world's busiest sea lanes off the east coast of Somalia, has seen increased activity.

The International Maritime Bureau's piracy reporting centre in Kuala Lumpur said there have been 147 attacks in the Gulf of Aden between January and September of this year.

With files from The Associated Press