The United States Geological Survey (USGS) on Sunday detected a first tremor, saying in an updated reading that the quake measured 6.3 on the Richter scale at a depth of 10 kilometers, 24 kilometers northeast of Sungjibaegam in North Korea.
South Koreas Joint Chiefs of Staff said in a statement that the seismic tremor had been detected near the Norths Punggye-ri nuclear test site.
China detected a second tremor, which it said was magnitude 4.6 and came at a depth of zero kilometers eight minutes after the first quake.
While South Korean officials said that the tremors were likely caused by a nuclear explosion, Japan said in certain terms that a nuclear test had occurred.
After examining the data, we concluded that it was a nuclear test, Japanese Foreign Minister Taro Kono said at a briefing after a meeting of Japans National Security Council.
Japans Ministry of Defense said it had dispatched at least three military jets from bases in Japan to test for radiation.
Absolutely unacceptable
Japanese Prime Minister Shinzo Abe had said earlier in the day a nuclear testwould be absolutely unacceptable.
If it forcibly conducted a nuclear test, its absolutely unacceptable. We have to strongly protest it, Abe said.
Hydrogen bomb
Just earlier on Sunday, North Koreas official Korean Central News Agency (KCNA) reported that the country had developed a hydrogen bomb that could be loaded into a new intercontinental ballistic missile.
The developments comeamid high tensions over the North Korean missile and nuclear programs.
South Korea, the US, and Japan have been unnerved by the Norths weapons programs, which Pyongyang has been defiantly advancing as countermeasures to their hostility.
On Tuesday, North Korea fired a Hwasong-12 missile reportedly capable of carrying a nuclear payload that traveled nearly 2,700 kilometers into the Pacific, including over Japan.
The violation of Japanese airspace sparked angry reactions from Japan and the US, as well as calls for restraint by other countries, including Russia and China. The launch also prompted a United Nations Security Council condemnation.
