The country said its sixth nuclear test was a perfect success. This was especially scary since hours earlier seismologists had detected an earth tremor.
Pyongyang said it had tested a hydrogen bomb which is many times more powerful than an atomic bomb.
According to analysts this might not be true, yet it is a source for concern.
South Korean officials said the latest test took place in Kilju County, where the North's Punggye-ri nuclear test site is situated.
Pyongyang said it had miniaturised a hydrogen bomb for use on a long-range missile, and North Korean leader Kim Jong- un was pictured with what state media said was a new type of hydrogen bomb.
State media said the device could be loaded on to a ballistic missile.
According to BBC, nuclear weapons expert, Catherine Dill, said it was not yet clear exactly what nuclear weapon design was tested.
Current information did not definitively indicate that a thermonuclear weapon had been tested but it appears to be a likely possibility at this point
There have been various responses to the development by world leaders. South Korean President Moon Jae-in reacted by saying North Korea's sixth nuclear test should be met with the "strongest possible" response, including new United Nations Security Council sanctions to "completely isolate" the country.
China also condemned the test. Japan's Chief Cabinet Secretary Yoshihide Suga said sanctions against North Korea should include restrictions on the trade of oil products.
Russia meanwhile said the test defied international law and urged all sides involved to hold talks, saying this was the only way to resolve the Korean peninsula's problems.
The head of the International Atomic Energy Agency, the global nuclear watchdog, described the test as "an extremely regrettable act".
Yukiya Amano said: