US President Donald Trump declared on Sunday that “appeasement with North Korea” will not work, after Pyongyang claimed it had successfully tested a missile-ready hydrogen bomb.

“North Korea has conducted a major Nuclear Test,” Trump said. “Their words and actions continue to be very hostile and dangerous to the United States.”

His comments came hours after the US Geological Survey picked up a 6.3 magnitude “explosion” in North Korea, which Pyongyang confirmed was a nuclear test, its sixth.

Earlier, Japan confirmed that North Korea conducted a nuclear test on Sunday, registering a formal protest with Pyongyang after a major explosion at the isolated nation's main test site.

“The government confirms that North Korea conducted a nuclear test after examining information from the weather agency and other information,” Japanese foreign minister Taro Kono told reporters.

He said the government registered a protest with the North Korean embassy in Beijing prior to the confirmation, calling any test “extremely unforgivable”.

“Today's nuclear test by the Democratic People's Republic of Korea (DPRK) is an extremely regrettable act,” International Atomic Energy Agency head Yukiya Amano said in a statement.

“This new test, which follows the two tests last year and is the sixth since 2006, is in complete disregard of the repeated demands of the international community.”

Trump last month threatened North Korea with “fire and fury” if it continued to threaten the United States, but he refrained from direct threats in his latest tweets.

“South Korea is finding, as I have told them, that their talk of appeasement with North Korea will not work, they only understand one thing!” he said.

Earlier, South Korea's military had expressed suspicion that North Korea had conducted its sixth nuclear test, after it detected a "strong earthquake."

The strong tremor was felt hours after Pyongyang claimed that its leader has inspected a hydrogen bomb meant for a new intercontinental ballistic missile.

South Korea's weather agency and the Joint Chief of Staff said an artificial 5.6 magnitude quake occurred at 12:29 pm local time, in Kilju, northern Hamgyong Province.

The US Geological Survey called the first quake an explosion with a magnitude 6.3.

Shortly after, Yonhap news agency said a second quake was detected with a magnitude 4.6 but South Korea's weather agency denied another quake occurred.

There was no word from the military in Seoul about the possible second quake.

North Korea conducted its fifth test last year in September. If confirmed, the latest test would mark yet another big step forward in North Korean attempts to obtain a nuclear-armed missile capable of reaching deep into the US mainland.

South Korea's presidential office said it will hold a National Security Council meeting chaired by President Moon Jae-in.

Islamabad condemns Pyongyang’s actions

Pakistan on Sunday condemned the reported nuclear test by North Korea.

A statement issued by the Foreign Office said, “Pakistan has consistently maintained that DPRK should comply with the UN Security Council resolutions and asked all sides to refrain from provocative actions.

Pakistan urges all sides to display utmost restraint and return towards the path of peaceful negotiated settlement of the issue, it added.

North Korea conducted two nuclear tests last year and has since maintained a torrid pace in weapons tests, including flight-testing developmental intercontinental ballistic missiles and flying a powerful mid-range missile over Japan.

Photos released by the North Korean government on Sunday showed Kim talking with his lieutenants as he observed a silver, peanut-shaped device that was apparently the purported thermonuclear weapon destined for an ICBM.

What appeared to be the nose cone of a missile could also be seen near the alleged bomb in one picture, which could not be independently verified and which was taken without outside journalists present. Another photo showed a diagram on the wall behind Kim of a bomb mounted inside a cone.

Aside from the factuality of the North's claim, the language in its statement seems a strong signal that Pyongyang will soon conduct its sixth nuclear weapon test, which is crucial if North Korean scientists are to fulfil the national goal of an arsenal of viable nuclear ICBMs that can reach the US mainland.

There's speculation that such a test could come on or around the Sept. 9 anniversary of North Korea's national founding, something it did last year.

As part of the North's weapons work, Kim was said by his propaganda mavens to have made a visit to the Nuclear Weapons Institute and inspected a “homemade” H-bomb with “super explosive power” that “is adjustable from tens (of) kiloton to hundreds (of) kiloton.”

North Korea in July conducted its first ever ICBM tests, part of a stunning jump in progress for the country's nuclear and missile program since Kim rose to power following his father's death in late 2011.

The North followed its two tests of Hwasong-14 ICBMs, which, when perfected, could target large parts of the United States, by threatening to launch a salvo of its Hwasong-12 intermediate range missiles toward the US Pacific island territory of Guam in August.

It flew a Hwasong-12 over northern Japan last week, the first such overflight by a missile capable of carrying nukes, in a launch Kim described as a “meaningful prelude” to containing Guam, the home of major US military facilities, and more ballistic missile tests targeting the Pacific.

Vipin Narang, an MIT professor specialising in nuclear strategy, said it's important to note that North Korea was only showing a mock-up of a two-stage thermonuclear device, or H-bomb.

“We won't know what they have until they test it, and even then there may be a great deal of uncertainty depending on the yield and seismic signature and any isotopes we can detect after a test,” he said.

To back up its claims to nuclear mastery, such tests are vital. The first of its two atomic tests last year involved what Pyongyang claimed was a sophisticated hydrogen bomb; the second it said was its most powerful atomic detonation ever.

It is almost impossible to independently confirm North Korean statements about its highly secret weapons program. South Korean government officials said the estimated explosive yield of last year's first test was much smaller than what even a failed hydrogen bomb detonation would produce.

There was speculation that North Korea might have detonated a boosted fission bomb, a weapon considered halfway between an atomic bomb and an H-bomb.

It is clear, however, that each new missile and nuclear test gives the North invaluable information that allows big jumps in capability.

A key question is how far North Korea has gotten in efforts to consistently shrink down nuclear warheads so they can fit on long-range missiles.

“Though we cannot verify the claim, (North Korea) wants us to believe that it can launch a thermonuclear strike now, if it is attacked. Importantly, (North Korea) will also want to test this warhead, probably at a larger yield, to demonstrate this capability,” said Adam Mount, a senior fellow at the Centre for American Progress.

North Korea is thought to have a growing arsenal of nuclear bombs and has spent decades trying to perfect a multistage, long-range missile to eventually carry smaller versions of those bombs.

South Korea's main spy agency has previously asserted that it does not think Pyongyang currently has the ability to develop miniaturised nuclear weapons that can be mounted on long-range ballistic missiles. Some experts, however, think the North may have mastered this technology.

The White House said that President Donald Trump spoke with Prime Minister Shinzo Abe of Japan regarding “ongoing efforts to maximise pressure on North Korea.”

The statement did not say whether the conversation came before or after the North's latest claim.

A long line of US presidents has failed to check North Korea's persistent pursuit of missiles and nuclear weapons. Six-nation negotiations on dismantling North Korea's nuclear program in exchange for aid fell apart in early 2009.

The North said in its statement Sunday that its H-bomb “is a multi-functional thermonuclear nuke with great destructive power which can be detonated even at high altitudes for super-powerful EMP (electromagnetic pulse) attack according to strategic goals.”

Kim, according to the statement carried by the state-run Korean Central News Agency, claimed that “all components of the H-bomb were homemade ... thus enabling the country to produce powerful nuclear weapons as many as it wants.”

In what could be read as a veiled warning of more nuclear tests, Kim underlined the need for scientists to “dynamically conduct the campaign for successfully concluding the final-stage research and development for perfecting the state nuclear force” and “set forth tasks to be fulfilled in the research into nukes.”

The two Koreas have shared the world's most heavily fortified border since their war in the early 1950s ended with an armistice, not a peace treaty.

About 28,500 American troops are deployed in South Korea as deterrence against North Korea.