Japanese Prime Minister Shinzo Abe said on Friday the nuclear test conducted by North Korea was not acceptable.
He added that Japan would liaise closely on the matter with the US and South Korea as the situation develops.
Abe said he had asked all relevant officials in his office and applicable ministries to garner more information about North Korea's testing of a nuclear device.
His instructions followed Japan Meteorological Agency (JMA) sensing an artificial earthquake centred in North Korea close to a earlier test site, which it believes may have been a nuclear test.
An artificial quake of magnitude 5 was detected at North Korea's main Punggye-ri nuclear test location.