Following North Korea's third nuclear test last week, Japanese officials have expressed concern about the reclusive state's possible progress toward fitting a miniaturized nuclear warhead atop a medium-range missile.
Pyongyang has already successfully test-fired and deployed the Rodong-1 ballistic missile, which has a range of 1,300 km.
A senior Foreign Ministry official said that if the North manages to equip the Rodong-1 with a nuclear warhead, it would pose a "significant threat" to the whole of Japan and necessitate a strengthening of the Japan-U.S. alliance, as well as enhanced nuclear deterrence measures by Washington.