Japan has confirmed a 6.3 quake in North Korea this afternoon was a nuclear test by the Kim Jong-un regime.

Japanese Foreign Minister Taro Kono says Japan has determined that North Korea had conducted its sixth nuclear test, a move described by Prime Minister Shinzo Abe as “absolutely unacceptable”.

South Korea’s Yonhap agency reported the north would make an “important announcement” at 06.30am GMT (4.30pm AEST).

According to a military report in South Korea, the test was estimated to have a yield of about 100 kilotons, four to five times stronger than the bomb dropped on Nagasaki in 1945 and 10 times stronger than the North’s fifth test, which was estimated to be about 10 kilotons.

One kiloton is equivalent to 1000 tons of TNT.

The tremor caused by the test was also 10 times as powerful as that from North Korea’s test a year ago and came just hours after it showed off what it called a hydrogen bomb capable of being mounted on a long-range missile.

The US Geological Survey said it had recorded a magnitude-6.3 earthquake that it described as a “possible explosion” in northeastern North Korea — near the site of Pyongyang’s past nuclear tests — at a depth of zero kilometres at noon Pyongyang time. The agency initially assessed it to be a magnitude-5.1 temblor.

Because earthquakes are measured using a logarithmic scale, a magnitude-6.3 quake would be 10 times as powerful as the one triggered by the North’s September 2016 nuclear test, which triggered a magnitude-5.3 earthquake, according to the USGS.

The Korea Meteorological Administration said that it had detected a revised magnitude-5.7 earthquake in the same area of North Korea, in what it described as likely being a “man-made” earthquake.

A spokesman for South Korea’s military said that it had strengthened its military posture in response to the likely nuclear test, adding that the Joint Chiefs of Staff had preliminarily assessed the incident to be a nuclear test.

A spokeswoman for the South Korean government said President Moon Jae-in had convened an emergency meeting of the National Security Council.

Japanese PM, Mr Abe, said his country would work together with the U.S., South Korea, China and Russia on a response to the apparent nuclear test.

“We can never accept it. We will need to make a strong protest,” Mr Abe said. North Korea’s September 2016 test had a likely yield of about 10 kilotons, larger than any of its previous four tests, but likely short of the hydrogen bomb that Pyongyang claimed that it detonated.

In this case, the magnitude-6.3 explosion would likely mean explosive power of around a megaton, according to Jeffrey Lewis, director of the East Asia Nonproliferation Program at the Middlebury Institute of International Studies in Monterey, California.

Earlier this morning, the North announced that Kim Jong-un witnessed a hydrogen bomb being mounted onto a new intercontinental ballistic missile while visiting the Nuclear Weapons Institute, North Korea’s state media said. The state media also published what experts said could be the North’s first photos of a purported hydrogen bomb.

Kim in the report boasted that all of the components of its thermonuclear weapon are homemade, insulating the nuclear-weapons program from sanctions and “enabling the country to produce powerful nuclear weapons, as many as it wants.”

While the claims couldn’t be immediately verified, the report, which didn’t specify the date of Kim’s visit, is likely to raise expectations of a sixth nuclear test or another long-range missile test by North Korea.

The White House said President Donald Trump spoke with Mr Abe to “discuss ongoing efforts to maximise pressure on North Korea.” It said the leaders “reaffirmed the importance of close co-operation between the United States, Japan, and South Korea in the face of the growing threat from North Korea.”

Mr Abe told reporters he and Mr Trump agreed to work closely together to increase pressure on North Korea. It was their third telephone discussion since Pyongyang launched a missile over Japan on Aug. 29.

North Korean has conducted a string of tests this year that have flaunted the country’s missile capabilities and menaced its neighbours. The North claims it can reach much of the continental U.S. with a long-range missile, though experts say the missile’s distance would be constrained by the size of any warhead that was mounted on it.

Scepticism remains as to whether the North can make a warhead small enough to fit on the tip of a missile, as well as whether any North Korean missile warhead would be robust enough to survive re-entry through Earth’s atmosphere.

In photos published by North Korean state media alongside its Sunday report, Kim was seen gesturing at a bulbous silver device that appeared capable of containing the two nuclear devices that would be necessary for a thermonuclear blast, as well as a possible outer shell for the device.

“There are a lot of things that are plausible about it...but we don’t have X-ray vision, so we can’t see into that,” said Melissa Hanham, a senior research associate at the Middlebury Institute of International Studies in Monterey, California, who said she believed the North was showing off the device in advance of a possible test.

A hydrogen bomb — technically known as a “thermonuclear weapon” — usually uses a smaller, primary atomic explosion to ignite a secondary, much larger blast. The first stage is based on nuclear fission — the splitting of atoms — and the second on nuclear fusion, which combines atoms, smashing them together and unleashing more energy. Additional stages can be added to increase its destructive force.

That makes the H-bomb more powerful than early nuclear weapons that typically used a single-stage blast based only on nuclear fission. Those weapons are known as “pure fission” devices and are thought to have been used in all of North Korea’s three previous nuclear tests, which it said involved atomic bombs.

According to the North’s report on Sunday, Kim watched a newly upgraded hydrogen bomb being loaded onto a new long-range missile. The bomb has explosive power that can range up to hundreds of kilotons, the North Korean report said.

Kim also specifically cited the possibility of an electromagnetic pulse, or EMP, attack. Fears of an EMP strike by North Korea have circulated for years among some US policy makers, though others have openly dismissed the possibility of Pyongyang launching such a strike.

In an EMP attack, North Korea would conduct a high-altitude nuclear detonation over the continental US. The detonation could emit a brief but powerful electromagnetic signal capable of disrupting swathes of the US electrical grid, experts say.

Ms Hanham said while such an attack was theoretically possible, the North’s goals made it more likely that Pyongyang would try to follow through on its threat to send a nuclear-tipped missile into a large American city.

Ultimately, she says, Sunday’s statement from Pyongyang “is a piece of propaganda meant to send a message to the US that they are a nuclear power that should be taken seriously and that we shouldn’t mess with them.” “They are trying to terrorise us,” Ms Hanham said.

The Wall Street Journal and AFP

-Agencies