North Korea says it has conducted a “successful” hydrogen bomb test, hours after two tremors were detected in the country.

North Korean state television said on Sunday that, “The hydrogen bomb test was a perfect success,” adding that the device was capable of being loaded onto long-range missiles.

The newsreader on the official Korean Central Television said the test had been of a bomb with “unprecedentedly large power,” and that it “marked a very significant occasion in attaining the final goal of completing the state nuclear force.”

The broadcaster showed an image of North Korean leader Kim Jong-un’s handwritten order for the test to be carried out at noon on September 3.

The announcement came after Japanese, South Korean, and US meteorological organizations said they had detected two tremors in North Korea.

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 Korea’s Joint Chiefs of Staff said in a statement that the seismic tremor had been detected near the North’s 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 Japan’s National Security Council.