MADRID - The United States has apologized to Spain for "what could seem an absurd situation" after it freed a missile-laden North Korean ship, earlier seized by Spanish troops and handed over to U.S. forces at U.S. request, Spain said on Thursday.

The ship, the So San, carrying Scud missiles to Yemen, was intercepted by Spain in the Arabian Sea on Monday and then handed over to U.S. forces, which later allowed it to continue on its course after deciding it was not breaking any laws.

"The Pentagon's number two, Paul Wolfowitz, called Defense Minister Federico Trillo on Wednesday to thank and praise Spain for the operation and to apologize for what could seem an absurd situation," a Spanish ministry of defense spokesman said.

The spokesman said authorities in Madrid were "a little surprised" by Washington's decision to release the ship as it had declared North Korea part of an "axis of evil" along with Iraq and Iran.

But Spanish Foreign Minister Ana Palacio, a staunch U.S. supporter, backed the decision.

"Once all the details were checked and the origin, destination and buyers were verified, and Yemen guaranteed that these missiles were not going to pass into terrorist hands, we were obliged to let the ship follow its course," Palacio told state radio. "And that is what the United States has done in this case."

The defense ministry spokesman said the incident would not affect Spain's attitude in future operations as it "took its duties seriously," a reference to Spanish support for the U.S. "war on terror."

The Spanish government rushed to support the United States after the September 11 attacks, saying it was inspired by its own fight against the violent Basque separatist group ETA.

Spain offered the use of its air bases to the United States immediately after September 11 and in recent months right-of-center Prime Minister Jose Maria Aznar and Palacio have expressed support for the U.S. campaign against Iraq.

Aznar is due to meet President Bush in Washington on December 18.