"That scale is to the level where anyone can say a hydrogen bomb test".
With Sunday's nuclear test, North Korea is seen drawing increasingly closer to becoming a full-fledged nuclear weapons state, which would further unsettle Northeast Asia's security landscape and escalate risks of a military standoff.
During a visit to the country's Nuclear Weapons Institute, North Korean leader Kim Jong Un "watched an H-bomb to be loaded into new ICBM [intercontinental ballistic missile]", KCNA reported.
The Comprehensive Test Ban Treaty Organisation (CTBTO) in Vienna said it had detected an "unusual seismic event" in North Korea that was larger than previous nuclear tests. But as Beijing argues, at least North Korea's pace of testing slows during talks, slowing the timeline for any potential crisis.
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. The last nuclear test Pyongyang conducted, which was almost a year ago, triggered a 5.3-magnitude seismological event.
Japan Meteorological Agency also observed a magnitude-6.1 tremor in North Korea, which showed a different waveform from a natural quake around 12:31 p.m. local (11:31 p.m. ET).
After the fifth nuclear test in September, USGS measured a magnitude of 5.3, while South Korean monitors said the blast caused a 5.0 magnitude quake.
North Korean leader Kim Jong Un provides guidance on a nuclear weapons program in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang September 3, 2017.
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While the type of bomb used and its size have not been independently verified, if true, the pariah state is a significant step closer to being able to fire a nuclear warhead to the U.S. mainland, as it has repeatedly threatened it could if provoked. That was revised up from an initial report of 5.6.
In Seoul, President Moon Jae-In called for new United Nations sanctions to "completely isolate North Korea" and said the South would discuss deploying "the strongest strategic assets of the USA military".
Japan confirmed that North Korea conducted a nuclear test, Foreign Minister Taro Kono said.
The United States Geological Survey detected an "explosion" with a magnitude of 6.3. In July this year, it apparently conducted its first two intercontinental ballistic missiles tests.
South Korea's military says North Korea is believed to have conducted its sixth nuclear test. The U.S. Treasury Department will prepare a package of sanctions that would "cut off North Korea economically", said Treasury Secretary Steven Mnuchin.
If the initial data holds, it would make it the most powerful weapon that North Korea has ever tested, according to U.S. officials. The chairmen of the joint chiefs of staff, General Jeong Kyeong-Doo and General Joseph Dunford, "agreed to prepare a South Korea-U.S. military counteraction and to put it into action at the earliest date". The administration also reported another quake in North Korea of magnitude 4.6, which it termed as a "collapse". A key question is how far North Korea has gotten in efforts to consistently shrink down nuclear warheads so they can fit on long-range missiles.
The latest nuclear test comes amid heightened regional tension following Pyongyang's two tests of ICBMs in July that potentially could fly about 10,000 km (6,200 miles), putting many parts of the USA mainland within range.