Four North Korean jets, including two MiG 29s, intercepted a US reconnaissance plane over the Sea of Japan on Sunday, the Pentagon said.

The North Korean jets 'shadowed' the RC-135S reconnaissance plane, which was on a routine intelligence mission, over international waters for about 20 minutes before breaking off, Pentagon spokesman Lieutenant Commander Jeff Davis said.

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

Davis said at one point one of the fighters 'locked on' to the US plane with its fire-support radar, an action that indicates intent to fire.

The US plane broke off its mission and returned to its home station at Kadena Air Base in Japan, he added.

Though Washington and Pyongyang have no diplomatic ties, the Bush administration plans to protest against North Korea's actions.

In 1969, a North Korean jet shot down a US Navy EC-121 surveillance plane, killing all 31 Americans aboard.

In April 2001, a Chinese fighter jet collided with a American Navy EP-3 plane, forcing it to land on China's Hainan Island. The Chinese pilot was killed and the American crew was detained for 11 days.