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North Korea appeared to carry out a sixth nuclear test Sunday, possibly of a hydrogen bomb more powerful than any device it has previously detonated, presenting President Donald Trump with an unprecedented challenge.
Monitors measured a 6.3-magnitude tremor near its main testing site, which South Korean experts reportedly said was nearly 10 times more powerful than the 10-kiloton test carried out a year ago.
The explosion came just hours after the North claimed to have developed a hydrogen bomb that could be loaded onto the country's new intercontinental ballistic missile.
Hydrogen bombs or H-bombs -- also known as thermonuclear devices -- are far more powerful than the relatively simple atomic weapons the North was believed to have tested so far.
Analysts' initial estimates of the yield from Sunday's test varied, ranging from 100 kilotons up to one megaton.
Either way, said Jeffrey Lewis of the armscontrolwonk website on Twitter, it was "a staged thermonuclear weapon" which represents a significant advance in its weapons program.
Chinese monitors said they had detected a second quake shortly afterwards of 4.6 magnitude that could be due to a "collapse (cave in)", suggesting the rock over the underground blast had given way.
Pyongyang has long sought the means to deliver an atomic warhead to the United States, its sworn enemy. A new test would be certain to infuriate Washington, Tokyo, Seoul, Beijing and others.
Tokyo said the tremor was a nuclear blast, which Japanese Prime Minister Shinzo Abe had said would be "absolutely unacceptable."
South Korean President Moon Jae-In summoned the National Security Council for an emergency meeting and Seoul's military raised its alert level.
Pyongyang triggered a new ramping up 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".
Analysts believe Pyongyang has been developing its weapons to give it a stronger hand in any negotiations with the US.
"North Korea will continue with their nuclear weapons programme unless the US proposes talks," Koo Kab-Woo of Seoul's University of North Korean Studies told AFP.
He pointed to the fact that Pakistan -- whose nuclear programme is believed to have links with the North's -- conducted six nuclear tests in total, and may not have seen a need for any further blasts.
"If we look at it from Pakistan's example, the North might be in the final stages" of becoming a nuclear state, he said.
- 'Super explosive power' -
Before the quake the official Korean Central News Agency said that leader Kim Jong-Un had inspected a miniaturised H-bomb that could be fitted onto an ICBM 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".
Pictures showed Kim in black suit examining a metal casing, with a shape akin to a peanut shell.
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.
The North carried out its first nuclear test in 2006, and successive blasts are believed to have been aimed at refining designs and reliability as well as increasing yield.
Its fifth detonation, in September last year, caused a 5.3 magnitude quake and according to Seoul had a 10-kiloton yield -- still less than the 15-kiloton US device which destroyed Hiroshima in 1945.
There was no immediate word from the North about Sunday's earthquake, but it said it would make an announcement at 3pm local time, 0630 GMT.
- 'Hundreds of kilotons' -
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.
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.
In Sunday's announcement before the earthquake, KCNA said the North's H-bomb had "explosive power that can be adjusted from tens to hundreds of kilotons depending on the target".
No H-bomb has ever been used in combat, but they make up most of the world's nuclear arsenals.