North Korea threatened on Thursday to strike US military bases in Japan, in response to the use of nuclear-armed US B-52 bombers in joint military drills with South Korea.

North Korea threatened on Thursday to strike US military bases in Japan, in response to the use of nuclear-armed US B-52 bombers in joint military drills with South Korea.

The threat by the Northern army came a day after Pyongyang condemned the B-52 flights as an "unpardonable provocation" and threatened military action if they continue.

“We cannot tolerate the US carrying out nuclear strike drills, setting us as targets, and advertising them as strong warning messages," a spokesman for the North's supreme army command said.

"The US should not forget that the Andersen base ... as well as naval bases at Japan's main island and Okinawa, are all within the range of our precision target assets," he said in a statement carried by the Korean Central News Agency.

"If the enemy threatens us with nuclear weapons, we will respond with stronger nuclear attacks," the North's army spokesman warned.

The Pentagon confirmed that B-52s, taking off from Andersen Air Force base in Guam, had flown over South Korea as part of annual joint exercises that Pyongyang insist are a rehearsal for invasion.

B-52s have taken part in South Korea-US joint exercises before, but the Pentagon said it had publicized their use this time to underline US commitment to defending its South Korean ally.