The U.S. Navy released the shipment of North Korean-made Scud missiles it seized, sending the vessel and its cargo on their way Wednesday to the original destination of Yemen.

White House spokesman Ari Fleischer said the United States had authority to stop and search the vessel, but not to seize it.

"There is no clear authority to seize the shipment," Fleischer told a news conference in Washington. "The merchant vessel is being released."

The decision followed high-level contact between Yemeni and U.S. officials including Vice President Dick Cheney and Secretary of State Colin Powell.

A senior Bush administration official, speaking on condition of anonymity, said Yemen has pledged not to purchase missiles in the future from North Korea, a secretive, communist country which Secretary of Defense Donald H. Rumsfeld has called the world's worst missile proliferator.

Even before the shipment was stopped, Yemen had agreed in principle to stop dealing with North Korea, but the agreement had not yet taken effect.

The official Saba news agency said the United States had assured Yemen that the shipment would be released as long as the Yemen-North Korea deal was concluded on legal basis.

Bush has ordered non-proliferation experts in the administration to determine how to make treaties and agreements that control the proliferation of nuclear, biological and chemical weapons to extend to the sale of missiles.

The Spanish navy had stopped the ship Monday off the Arabian peninsula, and U.S. authorities boarded it on Tuesday. The action came after intelligence officials watched the ship for weeks as part of an interdiction operation in the U.S.-led war on terrorism.

Spanish Defense Minister Federico Trillo said Wednesday the unflagged vessel was carrying 15 Scud missiles hidden in a cargo of cement.

"We became aware of the departure of the ship from North Korea that was carrying what we believe to be weapons of concern," Fleischer said. "This was a non-flagged vessel, which gave us further concern. And the vessel was destined for Yemen.

"We had a concern about what was on it. We had a concern before ascertaining, indeed, that it was going to Yemen, that it may have been heading for a nation that is a potential terrorist nation.

"As a result, the action that was taken, where the ship was stopped and boarded," Fleischer continued. "We have looked at this matter thoroughly. There is no provision under international law prohibiting Yemen from accepting delivery of missiles from North Korea."

The Bush administration in August imposed sanctions on the North Korean company Changgwang Sinyong Corp. for selling Scud missile parts to Yemen. At that time, U.S. authorities asked Yemen why it bought the parts; San'a apologized and promised not to do so again, two defense officials said Wednesday in Washington.

Under the U.S. sanctions, Changgwang Sinyong Corp. will be barred for two years from obtaining new individual export licenses through the Commerce or State departments for any controlled items. The sanctions have little practical effect, one official said, because there is so little commerce between the United States and North Korea.

Before the ship was freed, Yemeni Foreign Minister Abu Bakr al-Kerbi summoned U.S. Ambassador Edmund J. Hull to tell him the arms shipment was a "property of the Yemeni government and its armed forces and demanded that the United States should hand the shipment over to Yemen," Saba reported.

"The weapons contained in the shipment were to be used for defensive purposes as Yemen has no aggressive intentions toward any country, and owning such weapons would not harm the international peace and security," Saba quoted the official protest handed to Hull.

Yemeni officials had refused further details about the deal, including from what threat the Scud missiles were designed as a defense.

"Yemen has to defend itself, and this is our right," its foreign minister said. "Yemen does not trade in weapons." The Saba agency said the memo given to Hull claimed the shipment was part of a long-standing deal with North Korea. A senior Yemeni official, speaking on condition of anonymity, told The Associated Press on Wednesday that the Americans knew of the deal.

Trillo, Spain's Defense Minister, said the U.S. Navy had been planning to take the ship to Diego Garcia island, a British island leased to the United States as a military base.

Spain's role in the shipment's seizure earned the country a Yemeni protest memorandum as well in which San'a said the Spanish navy "didn't serve (to improve) relations between the two countries," Saba reported.

North Korea was silent Wednesday about the interception of the ship, but said it had the right to develop weapons to defend itself.

"It is necessary to heighten vigilance against the U.S. strategy for world supremacy and 'anti-terrorism war,"' the North's official newspaper, Rodong Sinmun, said in an editorial.

"All the countries are called upon to build self-reliant military power by their own efforts," the newspaper said. It was unclear whether the editorial was a response to the interception, as North Korea usually takes several days or longer to respond to international events.

The United States has stepped up its military presence in the region after the October 2000 attack on the USS Cole in the Yemeni port of Aden. The attack, blamed on Usama bin Laden's Al Qaeda network, killed 17 U.S. sailors.

Yemen, bin Laden's ancestral homeland, is a hotbed for Al Qaeda and other Muslim extremist militants who take refuge in the country's tribal strongholds. Its president has taken a number of anti-terrorism steps since the Sept. 11 attacks, including placing Islamic schools accused of teaching hate under government control.