WASHINGTON - A North Korean-owned ship was intercepted and boarded in the Arabian Sea on Monday by Spanish marines, who discovered at least a dozen Scud missiles under its cargo of cement, U.S. officials disclosed yesterday.

The destination was most likely Yemen, a U.S. official said.

After the Spanish marines requested U.S. help, sailors from an American warship boarded the vessel early yesterday.

The ship, which was not flying a flag and had no markings, made evasive maneuvers and ignored warnings, officials said.

It stopped only after shots were fired across its bow by the Spanish frigate Navarra.

The incident comes at a sensitive time in U.S. relations with both North Korea and Yemen. The Bush administration has pressured North Korea to halt a recently discovered uranium-enrichment program to fuel nuclear weapons.

The United States has also labeled North Korea a supplier of Scud missiles to several countries, in violation of international nonproliferation treaties.

Yemen has been a key regional ally for the United States in targeting Osama bin Laden's al-Qaida network. Though it has acquired Scud components from North Korea before, Yemen had pledged not to repeat the offense, officials said.

A White House official said no decision had been made on what action to take.

"We're working with other governments on next steps," the official said.

The Spanish marines who boarded the ship discovered the Scuds in containers under bags of cement. The Spanish were taking part in the international maritime anti-terrorism force.

Sailors from the amphibious assault ship USS Nassau, trained in explosive ordnance disposal, boarded the ship yesterday.

"They found Scud missiles; I'm not sure how many," one defense official said.

Another Pentagon official said that 20 containers holding the Scuds were found but that there might be more under the estimated 40,000 sacks of cement.

"The crew was North Korean," the official said. "There were about 20, and they were detained. As far as I know, they're still on board."

The ship had been tracked by U.S. intelligence since it left North Korea in mid-November, the official said.

In August, the United States imposed sanctions on North Korea after determining that it had transferred Scud missile components to Yemen during the Clinton administration.

The Bush administration chose not to impose sanctions against Yemen at the time because of its help in the war on terrorism and its commitment not to import more missiles from North Korea.

North Korea, which one official called a "habitual serial proliferator" of missiles in violation of international mandates, has been sanctioned by the United States at least seven times in the past decade.

Yemen was the site of the October 2000 terrorist bombing of the USS Cole that killed 17 sailors and wounded 39 others. U.S. officials have expressed concern that terrorist elements are operating in that area.

Last month in Yemen, the CIA used an unmanned Predator drone aircraft to fire a missile that killed a top al-Qaida leader and five others.