The move was part of the U.S. Air Force's efforts to check its military capacity in the Asia-Pacific region, they added.

In January the U.S. flew its strategic bunker-busting B-52 bomber across the skies of South Korea in response to North Korea's fourth nuclear test.

The B-52 Stratofortress, armed with long-range nuclear air-to-ground missiles, conducted a low-level flight over the U.S. Osan Base, some 55 kilometers south of the capital Seoul.

The June flight of the B-52 bombers appeared to be detected by North Korea.

Pyongyang insisted that a formation of B-52s was busy conducting a "nuclear bomb-dropping drill" on June 17.

On June 23, North Korea claimed the successful launch of what it called a Hwasong-10 intermediate range ballistic missile, known as the Musudan missile to other countries, saying that it has the capacity to strike U.S. forces in the Pacific region. (Yonhap)