WASHINGTON (CNN) # A U.S. Air Force RC-135 reconnaissance aircraft was intercepted over the Sea of Japan on Sunday by four armed North Korean MiG fighter jets, U.S. military sources said.

At least two of the planes were MiG-29s. The two other fighters were thought to be MiG-23s.

U.S. military sources said Monday that the Air Force plane was in international airspace about 150 miles [240 kilometers] off the Korean peninsula when the MiGs approached and flew alongside for 20 minutes, at some points coming within less than 400 feet of the U.S. plane.

One of the North Korean fighters locked its targeting radar onto the RC-135.

The RC-135 is a modified version of the military C-135 cargo plane, which is based on the Boeing 707 commercial airliner.

The Air Force plane returned to its base in Okinawa, Japan, without incident. U.S. officials say it was the first incident of a U.S. plane being intercepted by North Korean aircraft in more than 30 years.

The previous incident occurred in 1969, when a North Korean fighter shot down a U.S. EC-121 reconnaissance aircraft over the Sea of Japan, killing more than 30 U.S. airmen, according to a Pentagon official.