Officials in Japan and South Korea have concluded that the tremors detected in North Korea were a nuclear explosion, marking the sixth atomic test by Pyongyang since 2006.
Hours after the North's sixth nuclear detonation, an announcer on its official Korean Central Television declared "the hydrogen bomb test was a perfect success".
North Korea has boasted it has successfully tested an advanced hydrogen bomb that can be mounted onto an inter-continental ballistic missile.
Neither claim could be independently verified.
Newsreader Ri Chun-hee said the device that was successfully tested is the same as the one in a photo released by state media earlier.
North Korean officials identified the device as a two-stage, ICBM-mateable thermonuclear bomb.
Ri Chun-hee said no radiation was released, nor was there any negative environmental impacts from the test.
It comes as Japanese and South Korean officials confirmed that a shallow, 6.3 magnitude earthquake in North Korea had been a nuclear test.
"After examining the data we concluded that it was a nuclear tests," Foreign Minister Taro Kono said at a briefing broadcast by public broadcaster NHK following a meeting of Japan's National Security Council.
Second earthquake detected
China's Earthquake Administration said it detected a second quake in North Korea of magnitude 4.6, which it termed as a "collapse".
The administration's website said the second quake, measured at a depth of zero kilometres, came eight minutes after the firstquake, which it said was a "suspected explosion".
The co-ordinates of the two quakes were almost identical, according to figures provided by the administration.
The first quake struck 75km north northwest of Kimchaek.
Previous recent tremors in the region have been caused by nuclear tests, which if the case this time round, is bound to increase the tension hours after US President Donald Trump and Japanese Prime Minister Shinzo Abe talked by phone about the "escalating" nuclear crisis.
Witnesses in the Chinese city of Yanji, on the border with North Korea, said they felt a tremor that lasted roughly 10 seconds, followed by an aftershock.
South Korea mobilises nuclear crisis reponse team
The South Korean military says the first earthquake was man made, suggesting the rogue nation had conducted a sixth nuclear test.
It said the quake near the country's known nuclear test site Punggye-ri has been upgraded to 6.3 from 5.6 in magnitude.
The Yonhap news agency says the country’s nuclear crisis response team has mobilised.
North Korea says it has H-bomb missile warhead
Nuclear-armed Pyongyang has long sought the means to deliver an atomic warhead to the United States, its sworn enemy.
Questions remain over whether it has successfully miniaturised its weapons, and whether it has a working H-bomb, but the official Korean Central News Agency earlier said that leader Kim Jong-Un had inspected such a device at the Nuclear Weapons Institute.
It was a "thermonuclear weapon with super explosive power made by our own efforts and technology", KCNA cited Kim as saying, and "all components of the H-bomb were 100 percent domestically made".
US threatens fire and fury
North Korea triggered a new escalation of tensions in July, when it carried out two successful tests of an ICBM, the Hwasong-14, which apparently brought much of the US mainland within range.
It has since threatened to send a salvo of rockets towards the US territory of Guam, and last week fired a missile over Japan and into the Pacific, the first time time it has ever acknowledged doing so.
US President Donald Trump has warned Pyongyang that it faces "fire and fury", and that Washington's weapons are "locked and loaded".
Trump spoke by telephone to Japan's Prime Minister Shinzo Abe to discuss the need to "maximize pressure on North Korea" in the face of the "growing threat" it presented, according to a White House readout of the call, without specifying when it took place.
The North has repeatedly claimed that it has a thermonuclear weapon, which can be far more powerful than other nuclear devices.
When it carried out its fourth nuclear test, in January 2016, it said it was a miniaturised H-bomb, but scientists said the six-kiloton yield achieved then was far too low.
When it carried out its fifth test, in September 2016, it did not say it was a hydrogen bomb.
Yang Moo-Jin of the University of North Korean Studies in Seoul, told AFP the latest KCNA report "carries a strategic message" that Pyongyang "will push for a nuclear face-off with the US as an equal".
But the Washington-based Institute for Science and International Security urged caution in response to the North's latest assertions, saying they should be viewed as "an intention rather than a reality".
"NK can build any H-bomb (two stage) model it wants and call it whatever it wants but that does not make it real," it said on Twitter, adding the picture "looks like a model in a room for models, not for loading nuclear devices into re-entry vehicles".
Actually mounting a warhead onto a missile would amount to a significant escalation on the North's part, as it would create a risk that it was preparing an attack.
H-bomb has 'explosive power'
The North Korean leadership says a credible nuclear deterrent is critical to the nation's survival, claiming it is under constant threat from an aggressive United States.
It has been subjected to seven rounds of United Nations Security Council sanctions over its nuclear and ballistic missile programmes, but always insists it will continue to pursue them.
Its first nuclear test was in 2006, and successive blasts are believed to have been aimed at refining designs and reliability as well as increasing yield.
In September last year, North Korea boasted its fifth atomic test was "most powerful to date", according to Seoul, with a 10-kiloton yield -- still less than the 15-kiloton US device which destroyed Hiroshima in 1945.
Atomic or "A-bombs" work on the principle of nuclear fission, where energy is released by splitting atoms of enriched uranium or plutonium encased in the warhead.
Hydrogen or H-bombs, also known as thermonuclear weapons, work on fusion and are far more powerful, with a nuclear blast taking place first to create the intense temperatures required.
The North's H-bomb had "explosive power that can be adjusted from tens to hundreds of kilotons depending on the target", KCNA said Sunday, claiming technological advances "on the basis of precious successes made in the first H-bomb test".
No H-bomb has ever been used in combat, but they make up most of the world's nuclear arsenals.
Melissa Hanham of the Middlebury Institute for International Studies in California said the images could not be proved real of themselves.
"We don't know if this thing is full of styrofoam, but yes, it is shaped like it has two devices," she said on Twitter.