Analysts have warned other nations to treat the matter seriously, adding that the regime's nuclear programmes are picking up speed.
North Korea conducted its fifth test a year ago in September.
Earlier, the Kim Jong-un led government had claimed that they have successfully managed to miniaturise a nuclear weapon that can be loaded on to a missile.
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.
North Korea said it tested a hydrogen bomb that could be mounted on a missile and that its biggest-ever nuclear detonation had been a "perfect success". State media said the device could be loaded on to a ballistic missile.
There was no immediate announcement from the North about today's quake.
A series of recent missile tests has caused growing global unease.
While the United States has virtually no trade with the North, the burden of sanctions as described by Trump and Mnuchin would fall heavily on China, which is Pyongyang's sole major ally and buys about 90 percent of North Korean exports.
Naturally, this statement wasn't what people wanted to hear from an expert on nuclear weapons. There is also scepticism about the North's claims to have developed a hydrogen bomb. This is an important component of the North's deterrent strategy: It feels it must showcase that it can do significant damage to a city on the US mainland to change USA calculus on military action or attempted regime change in North Korea.
Nuclear weapons expert Catherine Dill told the BBC it was not yet clear exactly what nuclear weapon design was tested.
She said: "This is now even more pressing".
What has the reaction been?
Those readings would put the size of the explosion at several times larger than previous North Korean tests.
How can the world respond?
"(North Korea) ignored repeated warnings by the global community and forcibly conducted a nuclear test.
How did news of the test emerge? South Korea's weather agency said the artificial quake caused by the explosion was five times to six times stronger than tremors generated by the North's previous five tests. The test reportedly set off a man-made quake in the country's northeastern region, which was reportedly felt in China.