President Moon Jae-in convened a National Security Council (NSC) session Sunday, shortly after South Korea's weather agency detected a magnitude 5.7 natural disaster from North Korea's nuclear test site in a sign of another strategic provocation.
Hans Kristensen, a nuclear weapons expert at the Federation of American Scientists, said the North probably will need to do more tests before achieving a functioning hydrogen bomb design.
South Korea's military said earlier Sunday that North Korea is believed to have conducted its sixth nuclear test after it picked up seismic waves measuring 5.6.
The test is a direct challenge to U.S. President Donald Trump, who hours earlier had talked by phone with Japanese Prime Minister Shinzo Abe about the "escalating" nuclear crisis in the region and has previously vowed to stop North Korea developing nuclear weapons that could threaten the United States.
Sunday's announcement came hours after a large natural disaster that appeared to be man-made was detected near the North's known nuclear test site, indicating that the reclusive country had conducted its sixth nuclear test.
The Arms Control Association said the explosion appeared to produce a yield in excess of 100 kilotons of TNT equivalent, which it said strongly suggests the North tested a high-yield but compact nuclear weapon that could be launched on a missile of intermediate or intercontinental range.
China, the North's sole remaining major ally, issued a "strong condemnation" of the test, which overshadowed the opening of the BRICS summit in Xiamen by leader Xi Jinping.
South Korea's Joint Chiefs of Staff said in a statement that the military had "detected a man-made natural disaster near Punggye-ri and is analyzing whether it was a nuclear test". Kim's regime has said it won't give up its nuclear weapons and missile program until the US drops its "hostile" policies such as joint military drills with South Korea that ended last week.
Experts are sceptical about the claim that Pyongyang has mastered hydrogen technology, but it is nearly impossible to independently confirm statements about its highly secret weapons programme. State media reported that the test left no trace of radioactive material.
Some former administration officials and experts around Washington have floated the idea of a pre-emptive attack on the condition that the US and South Korea detect clear signs of the North's immediate missile launch. South Korea's military said it has strengthened its monitoring and readiness while mulling a variety of possible responses that could be executed in collaboration with the U.S.
In its report, KCNA (via Xinhua, China's official news agency) also said the North Korean leader "guided the work for nuclear weaponization on the spot", as part of "the final-stage research and development for perfecting the state nuclear force".
A US official who studies North Korea's military and politics said it was too early to determine if atest supported the North's claim that it had succeeded in developing a thermonuclear weapon, "much less one that could be mounted on an ICBM and re-enter Earth's atmosphere without burning up".
Any potential USA move for military action, however, would prompt resistance from the Moon Jae-in government, which has been calling for dialogue as a first and foremost solution to the crisis.
North Korea 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 White House said Trump spoke with Abe regarding "ongoing efforts to maximize pressure on North Korea".
The nuclear test is understood to be the largest yet and follows two last year, as well as a string of missile tests this year. But the statement did not say whether the conversation came before or after the North's latest test.