The secretive regime Sunday tested a hydrogen bomb that can fit on an intercontinental ballistic missile.

The nation claimed the test was a "perfect success", after a 6.3 magnitude earthquake was registered by the U.S. Geological Survey. The tremors originated from North Korea's nuclear test site in North Hamgyong Province.

Norwegian seismological observatory NORSAR estimated the explosive yield of the device was 120 kilotons TNT. If correct, that explosion is 6 times the explosive yield of the bomb dropped on Nagasaki in 1945.

"The national security team is monitoring this closely" - White House Press Secretary Sarah Huckabee Sanders.

South Korean military has detected indications Pyongyang is readying for another long-range missile launch, South Korea's central news agency reported Monday.

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