The United States is still haunted by a nightmare of 2006, says an insider of the Defense Ministry. But the "nightmare" he refers to is not the first nuclear test by North Korea in October that year, but rather an internal report compiled by the Japanese government a month earlier on "the possibility of domestically producing nuclear weapons." North Korea carried out its third nuclear test on Feb. 12, 2013.

The U.S. is nervous about Japan, according to the insider. It is not publicly known that the Foreign Ministry has been conducting studies clandestinely on potential development of nuclear arms.

Two days after the latest nuclear test by the North, U.S. President Barack Obama told Prime Minister Shinzo Abe in a telephone meeting that there would be no change whatsoever in America's commitment to defend Japan, including nuclear deterrence through its nuclear umbrella over Japan.