WASHINGTON -- Four armed North Korean fighter jets intercepted a U.S. reconnaissance plane over the Sea of Japan and one of the Korean jets used its radar in a manner that indicated it might attack, U.S. officials said Monday.

Lt. Cmdr. Jeff Davis, a Pentagon spokesman, said it was the first such incident since April 1969 when a North Korean plane shot down a U.S. Navy EC-121 surveillance plane, killing all 31 Americans aboard.

The most recent crisis involving U.S. reconnaissance aircraft was in April 2001 when a Chinese fighter jet collided with a Navy EP-3 plane, forcing it to make an emergency landing on China's Hainan Island. The fighter pilot was killed and the American crew was detained for 11 days.

The latest incident happened Sunday morning, Korean time, and there was no hostile fire, Davis said.

A dispute between the United States and North Korea over nuclear weapons development increased last week when North Korea restarted a 5-megawatt reactor that could produce plutonium for such weapons. North Korea said Saturday that nuclear war could break out at "any moment."

In the Sunday incident, Davis said, North Korean planes "shadowed" the American plane over international waters for about 20 minutes before breaking off.

Two North Korean MiG29 fighters and two other aircraft that Davis said appeared to be MiG23 fighters intercepted the Air Force RC-135S reconnaissance plane, which Davis said was conducting a routine intelligence mission over the Sea of Japan about 150 miles off North Korea's coast.

The closest the fighters came was about 50 feet, Davis said.

He did not know whether there was any communication between the North Korean and American crews.

At one point one of the fighters "locked on" to the U.S. plane with its fire-support radar, Davis said. This is an action that would indicate a possible intent to fire, although in this case there was no hostile fire.

The U.S. plane broke off its mission and returned to its home station at Kadena Air Base in Japan, Davis said.

The American plane is a highly specialized version of the RC-135 series of reconnaissance planes. This version, nicknamed "Cobra Ball," is loaded with electronic receivers and features large circular windows in the fuselage for the photography of foreign ballistic-missile tests at long range. The intelligence equipment aboard includes multiple infrared telescopes.

The RC-135 planes are modified Boeing 707s.

The incident happened amid heightened tensions between the United States and North Korea. The two countries have no formal diplomatic relations, and North Korea frequently complains that joint U.S.-South Korean military exercises are a prelude to a U.S.-led invasion.

The U.S. Air Force regularly flies U-2 spy plane missions to monitor North Korea's military, including its nuclear facilities.