MADRID, Spain — Spain said Friday it would not free two captured pirates as demanded by their fellow hijackers who are holding a Spanish trawler and 33 crew members off the coast of Somalia.

The trawler's captain told Spanish media Thursday that the pirates on board had threatened to start killing the hostages.

Deputy Defence Minister Constantino Mendez said Friday the two Somali men were captured in connection with the hijacking of the Spanish-registered tuna boat Alakrana on Oct. 2 in the Indian Ocean and brought to Madrid.

He told Spanish National Radio: "The situation is not negotiable."

However, he seemed to leave open a possibility of transferring them to the court system of another country.

Pirates holding the Alakrana took three crew members ashore to Somalia on Thursday, the Defence Ministry said.

The wives of two crew members who spoke to their husbands Thursday said the pirates are demanding the release of the two in custody in Madrid as a condition for letting the ship and its crew go.

On Thursday night, the skipper of the Alakrana, Ricardo Blach, told Spanish television the heavily armed pirates on board had threatened to kill the three crew members taken ashore if there was no progress in freeing the two men.

"They told us an hour ago that if there is no movement relating to those who are in Spain, then they would begin by killing those three in three days' time, and then they would take another three, and so on," Blach said.

On Friday, relatives of the crew issued an urgent appeal for the Spanish government to free the detained pirates. "Otherwise they will be jeopardizing the lives of our loved ones," they said in a statement released in the Basque town of Bermeo, where the Alakrana is based.

The company that owns the Alakrana, Echebastar Fleet, urged the government to "facilitate the departure of the two Somalis detained in Spain, taking urgent measures."

Mendez ruled out freeing the two. But when asked if they might be transferred to an African country, similar to a case in May, he seemed to suggest that was an option.

"One can discuss issues of jurisdiction at length. They have many angles and law is not mathematics. Therefore, it is something that is open to differing opinions," Mendez said.

In May, Spanish naval forces caught seven young pirates trying to hijack a Panamanian-flagged ship in the Gulf of Aden. Courts here initially considered bringing them to Madrid, but ultimately turned them over to Kenya under an anti-piracy agreement with the European Union.