South Korea's President, Moon Jae-in, has called a meeting of his National Security Council.
The office says U.S. National Security Adviser H.R. McMaster spoke with his South Korean counterpart, Chung Eui-yong, for 20 minutes in an emergency phone call about an hour after the detonation.
The Tokyo government has registered a protest with the North Korean embassy in Beijing, he said.
Earlier Sunday, Pyongyang claimed it had developed a hydrogen bomb that can be mounted on the tip of an intercontinental ballistic missile - and threatened a high-altitude nuclear explosion that experts fear could wipe out USA electrical networks. "Possible explosion, located near the site where North Korea has detonated nuclear explosions in the past", read a statement on the USGS website.
In this undated image distributed on Sunday by the North Korean government, shows North Korean leader Kim Jong-un at an undisclosed location.
North Korea a year ago conducted its fourth and fifth nuclear tests, saying the fourth in January 2016 was a successful hydrogen bomb test, although outside experts say the claim has not been proven.
The blast first measured in at 5.6 on the Richter scale but has since been revised up to a much stronger 6.3.
Japan's defense minister says the larger magnitude of the quake suggests "capability significantly exceeding the last one".
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North Korean leader Kim Jong-un inspected the loading of an "H-bomb into the ICBM during his visit to the Nuclear Weapons Institute, according to the Korean Central News Agency (KCNA)". North Korea wants to demonstrate it knows what makes a credible nuclear warhead. There was no word from the military in Seoul about the reported second quake. NORTH Korea has confirmed it has tested a thermonuclear bomb with "complete success".
Earlier reports said there were indications suggesting the North had been preparing for its sixth nuclear test.
It is clear, however, that each new missile and nuclear test gives the North invaluable information that allows big jumps in capability.
South Korea, the U.S., and Japan have been unnerved by the North's weapons programs, which Pyongyang has been defiantly advancing as countermeasures to their hostility.
It has since tested two intercontinental ballistic missiles in July and last month, fired a potentially nuclear-capable missile over northern Japan.
Donald Trump has previously threatened North Korea with "fire and fury like the world has never seen" if they continued threatening his country and its territory of Guam.
South Korea and Japan are gathering and analysing data to confirm details of the test, which Japanese Prime Minister Shinzo Abe said could not be tolerated.
There was no independent confirmation of the North Korean reports, and Western officials did not comment directly on the claims.
Sunday's events could also change the equation in Washington.