North Korea appeared to carry out a sixth nuclear test Sunday, with seismic monitors measuring an "explosion" of 6.3 magnitude near its main test site, sending tensions over its weapons ambitions to new heights.
The apparent test came just hours after it claimed to have developed a hydrogen bomb that could be loaded into the country's new intercontinental ballistic missile.
The South's Joint Chiefs of Staff said in a statement that the seismic tremor was detected near the North's Punggye-ri test site.
United States Geological Survey recorded the magnitude at 6.3 -- larger than any previous test.
Jana Pursely, a USGS geophysicist, told AFP: "It's an explosion rather than an earthquake."
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 said before the quake 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".
Pictures showed Kim in black suit examining a metal casing, with a shape akin to a peanut shell.
North Korea 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".