Published: March 04, 2003

Four North Korean fighter jets intercepted an unarmed United States Air Force spy plane on a surveillance mission over the Sea of Japan on Saturday, and came within 50 feet of the American aircraft, military officials said today.

The fighters shadowed the spy plane, an RC-135S Cobra Ball, for 22 minutes in international airspace about 150 miles off the North Korean coast, said Lt. Cmdr. Jeff Davis, a Pentagon spokesman. No shots were fired, officials said.

Military officials said they had no indication that United States fighter jets were scrambled to protect the routine flight by the reconnaissance plane before it broke off its mission and returned safely to its home base at Kadena Air Base in Japan.

But officials said the reconnaissance missions by the RC-135S, which tracks ballistic-missile launchings, would continue with armed American fighter escorts. That raised the possibility of an escalation of tensions, officials said.

''This is serious stuff,'' said a senior American official who deals frequently with North Korea policy. ''It's worrisome because they are creating their own drumbeat.''

The incident was the latest in a series of provocations by North Korea in what Washington says is its quest to build a significant nuclear arsenal in defiance of American and international protests.

Last week, American intelligence officials said that North Korea had restarted a reactor at its primary nuclear complex. Over time, the reactor could provide a steady source of plutonium for nuclear weapons.

At the same time, North Korea also conducted its first missile launching in three years, firing a rocket into the sea between Korea and Japan that landed in North Korean waters. Just days earlier, a North Korean fighter plane swooped over the Yellow Sea off the South Korean west coast for two minutes.

But until the weekend incident, the episodes had not involved direct encounters between American and North Korean forces. It was the most serious aerial confrontation between the countries since North Korea shot down an unarmed American reconnaissance plane in 1969, killing 31 American airmen.

The incident occurred late Saturday night -- Sunday morning in Korea -- and military officials gave no explanation for their delay in discussing the event.

Pentagon officials initially said that one of the armed North Korean fighters -- two MIG-29's and two MIG-23's -- had 'locked on' to the Cobra Ball with its fire-control radar, which is considered preparation for firing an air-to-air missile. But officials later pulled back from that assessment until they could review data tapes of the incident.

One senior military official said the distinction was marginal, since at least one of the fighters was just moments away from that crucial targeting step when the American jet aborted its mission and flew toward Japan. ''This had to be well planned on their part,'' the military official said.

Military officials said there were no radio communications between the American and North Korean aircraft during the tense standoff. A senior military officer said tonight that the American flight crew reported that one of the North Korean pilots gestured to them to leave the area, and ''he was waiving at them to get out of there.''

A senior administration official said, ''There was some real concern for a few minutes on Saturday when they locked on. It'll be taken up with other countries in the region. There will be a protest filed with the North Koreans.''