Washington ? Four armed North Korean fighter jets intercepted a U.S. reconnaissance plane over the Sea of Japan and one 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 latest incident happened Sunday morning, Korean time, and there was no hostile fire, Davis said.

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.

U.S. officials have said they have no plans to invade North Korea but are growing increasingly concerned about the North’s reactivation of a nuclear reactor that is part of a suspected weapons program. Washington believes Pyongyang already has one or two nuclear weapons.

The dispute 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.”

At the State Department, spokesman Richard Boucher said he had no comment on the latest incident beyond what the Pentagon reported. He did express concern about North Korea’s nuclear program.

“All of North Korea’s neighbors have made clear that this kind of development would be unwelcome and have serious consequences for them,” Boucher said.

In the Sunday incident, Davis said, the four 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. He also did not know how many crew members were aboard the RC-135, although the standard number is 17, including two pilots.

At one point one of the fighters used a radar system to “acquire” the U.S. plane, meaning it identified it as a target. The Pentagon initially reported that the Korean fighter then “locked on” to the RC-135, meaning it engaged a separate fire-support radar to provide the targeting data needed to strike the American plane with a missile. Later, Davis said officials were reviewing available data to confirm that the second “lock on” step was taken.

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.

Davis said missions of this kind had been flown in that area off the North Korean coast for many years without incident.

The RC-135 planes are modified Boeing 707s.

The United States and North Korea 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.