The hydrogen bomb test follows North Korea's August 29 launch of a ballistic missile that landed in the Pacific off northern Japan.
There was no independent confirmation that the detonation was a hydrogen bomb rather than a less powerful atomic weapon of the kind Pyongyang has tested in the past.
This would be North Korea's first test of a nuclear weapon since President Donald Trump took office.
The tremor struck within a kilometre of the site of a magnitude-5.3 "nuclear explosion" from September a year ago, according to the United States Geological Survey (USGS).
Trump tweets that North Korea "has become a great threat and embarrassment to China, which is trying to help but with little success".
South Korea's Joint Chief of Staff said all South Korean troops have been put on high alert, with the North scheduled to commemorate its founding anniversary later this week, reports Yonhap News Agency.
"Despite the anger of the Chinese public toward North Korea's new nuclear test, we should avoid resorting to rash and extreme means by imposing a full embargo on North Korea", it said in an editorial.
South Korea's military says North Korea is believed to have conducted its sixth nuclear test.
N.Korea Can Fire Missile from Anywhere Now
History reveals that USA has been at war with North since the 1950-53 conflict that ended up in a truce rather than peace treaty. North Korea has been under United Nations sanctions since 2006 over its ballistic missile and nuclear programmes.
"North Korea will continue with their nuclear weapons programme unless the United States proposes talks", Koo Kab-Woo of Seoul's University of North Korean Studies told AFP. Kim's regime has said it won't give up its nuclear weapons and missile program until the United States drops its "hostile" policies such as joint military drills with South Korea that ended last week. China has resisted doing anything that would lead to the collapse of Kim's regime, in part to avoid destabilising its economy and seeing the United States military gain influence in a unified Korea.
The broadcaster carried images of leader Kim Jong-Un, the third generation of the family to rule the Democratic People's Republic of Korea, as the country is officially known, sitting at a circular table with a general and three other officials.
The annual military exercises in the South always lead to a sharp rise in tensions, and the North's fifth nuclear test on September 9 previous year also followed the annual war games. The North's official news agency, KCNA, also released the photos, which were clearly meant to be seen by a global audience. The North in July had demonstrated for the first time that it has - or is very close to having - an operational ICBM, though experts still believe it could at best reach Chicago and will probably require another year or two to flawless.
This is an important component of the North's deterrent strategy: It feels it must showcase that it can do significant damage to a city on the US mainland to change USA calculus on military action or attempted regime change in North Korea.
In a Saturday evening phone call - which Trump made from aboard Air Force One as he returned home to Washington from his visit to storm-battered Texas and Louisiana - the two leaders discussed "ongoing efforts to maximize pressure on North Korea", according to the White House. It urged North Korea to "stop taking erroneous actions that deteriorate the situation".
"We strongly urge (the) North Korea side to face up to the firm will of the global community on the denuclearization of the peninsula, abide by relevant resolutions of the UN Security Council, stop taking wrong actions that exacerbate the situation and are not in its own interest, and return to the track of resolving the issue through dialogue", the Chinese Foreign Ministry said.
- The explosion created a magnitude-6.3 tremor, making it the most powerful weapon Pyongyang has ever tested.
In the 12 days since Washington praised Pyongyang's "restraint" and held out the prospect of early talks, the North has fired three short-range missiles, sent another one soaring over Japan and detonated what appears to be a full-fledged thermonuclear device. When asked whether there may be a change to Seoul's policy vis-a-vis Pyongyang - that is, a policy of engagement, dialogue AND pressure - a senior Blue House official said although there will not be a shift of policy in the big picture any kind of dialogue between South and North Korea at this point seem highly unlikely.
- US bombers drill over Korean peninsula days after latest missile launch
- USA orders Russian Federation to close 3 diplomatic buildings
- Merkel, rival Schulz spar over Turkey in TV election debate
- United Kingdom minister: European Union commission criticism of Brexit talks was 'silly'
- Energy Secretary Rick Perry taps emergency oil reserve in wake of Harvey
- China says Trump's trade threat over N. Korea 'unacceptable'
- Last-ditch effort to restore Stormont Executive in Northern Ireland
- Standings, bubble drivers entering final regular-season race — NASCAR playoffs
- Prince William, Duchess Catherine Expecting 3rd Child: Palace
- EI du Pont de Nemours and Company