But experts who studied the impact of the quake caused by the explosion - measured by the US Geological Survey at magnitude 6.3 - said there was enough strong evidence to suggest the reclusive state has either developed a hydrogen bomb or was getting very close. "The H-bomb test was carried out to examine and confirm the accuracy and credibility ... of [an] H-bomb to be placed in the payload of the [intercontinental ballistic missile]".
In a high-level national security meeting, South Korean President Moon Jae-in called the test "an absurd strategic mistake" that will lead to the global community further isolating Pyongyang. It says its weapons programmes are needed to counter U.S. aggression.
A map of the Sea of Japan.
The South's National Fire Agency, which operates an emergency hotline, said it had received 31 calls about buildings and the ground shaking, the first time that South Koreans had reported tremors after a North Korean nuclear detonation.
South Korea, China and Russian Federation all voiced strong criticism of the move.
The North has previously claimed to have successfully tested an H-bomb, after its fourth blast in January past year, but experts doubted the claim at the time, due to the relatively weak yield.
NORSAR, a Norwegian seismological observatory, said the explosive yield measured 120 kilotons TNT.
North Korea's sixth nuclear test was conducted as leaders from Russia, Brazil, India, and South Africa flew into China for the much-heralded BRICS summit of emerging economies, at which Chinese President Xi Jinping was due to give a speech.
"Today's nuclear test by the Democratic People's Republic of Korea (DPRK) is an extremely regrettable act". It's first test was in 2006, according to ABC News, followed by tests in 2009, 2013 and two tests in 2016.
Trump also spoke recently with South Korean President Moon Jae-in.
Hours before the latest test, North Korean state media published photographs of leader Kim Jong Un inspecting a peanut-shaped device that it said was a hydrogen bomb created to be loaded on a new ICBM.
"If countries want to do business with the United States, they obviously will be working with our allies and others to cut off North Korea economically", Mnuchin said on "Fox News Sunday".
A military response from the United States appears less likely than a decision by China to finally reach for its biggest gun - cutting the oil pipeline to North Korea.
This time, the North claims to have mastered a far more powerful hydrogen weapon.
"Obviously the test yesterday shows they are further along than everyone figured", said Flake, a Republican. The Kim regime has developed its weapons and nuclear program despite the measures that have crippled the economy and exacerbated periods of mass starvation.
And earlier this month, North Korean leader Kim Jong Un threatened to launch a strike against the Pacific island of Guam, but later backed off that threat.