Its fifth nuclear test in September 2016 was measured to be possibly North Korea's biggest detonation ever, but the quake it caused was still not believed to be big enough to demonstrate a thermonuclear test.
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).
The announcement was made three hours after the test.
"We will do our utmost to protect people's lives", Japanese Prime Minister Shinzo Abe said.
North Korea in July conducted its first ever ICBM tests, part of a stunning jump in progress for the country's nuclear and missile program since Kim rose to power following his father's death in late 2011.
When it carried out its fourth nuclear test, in January 2016, it said it was a miniaturised H-bomb, but scientists said the six-kiloton yield achieved then was far too low.
The country has previously gone against United Nations sanctions and conducted dozens of ballistic missile tests under Kim but this is one of the first time a projectile has been flown over mainland Japan.
One expert said the size of Sunday's detonation meant it was possible it could be a hydrogen bomb test.
Several national geological agencies detected unusual seismic activity near the location of Pyongyang's previous tests in the country's northeast at around that time. The last test, in September 2016, yielded one about as powerful as the bomb the United States dropped on Hiroshima in 1945.
While citizens of nearby Yanji in China and Vladivostok in Russian Federation both reported feeling quake tremors for several seconds during the first event, the second natural disaster was not however felt, two informed sources in Yanji told NK News. China said it had detected a second, 4.6 magnitude quake with near identical coordinates eight minutes later. The ministry urged North Korea to respect U.N. Security Council resolutions, Reuters reported. "The city air raid sirens started going off".
Trump and South Korean President Moon Jae-in spoke by phone on Saturday to discuss responses to North Korea's provocations, and Trump "provided his conceptual approval of planned purchases by South Korea of billions of dollars in American military equipment", according to a White House statement.
Japan said it had concluded there was a nuclear test.
Aside from the factuality of the North's claim, the language in its statement seemed a strong signal that Pyongyang was planning another nuclear weapon test, which is crucial if North Korean scientists are to fulfill the national goal of an arsenal of viable nuclear ICBMs that can reach the USA mainland.
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"This test is just another step towards such a goal".
Earthquakes triggered by North Korean nuclear tests have gradually increased in magnitude since Pyongyang's first test in 2006, indicating the isolated country is steadily improving the destructive power of its nuclear technology.
The 5.7 magnitude "artificial earthquake" that occurred in the Punggye-ri area at around 12:29 p.m.is "presumed" to be from the North's sixth nuclear test, the Joint Chiefs of Staff (JCS) announced.
Kim was also quoted as saying that North Korea can now produce as many powerful nuclear weapons "as it wants".
Another threat is deemed to be looming ahead as North Korean leader Kim Jong Un has been spotted inspecting a new, ultra explosive hydrogen bomb.
"Referring to tens to hundreds of kilotons, it doesn't appear to be talking about a fully fledged H-bomb".
'We don't know if this thing is full of styrofoam, but yes, it is shaped like it has two devices, ' she said on Twitter.
North Korea conducted it's 4th and 5th nuclear tests in January and September of a year ago.
Dressed in a black suit and flanked by his lieutenants, the Supreme Leader was seen examining a metal casing with two bulges that was apparently the purported weapon.
North Korea conducted two nuclear tests last year, the last almost a year ago, on the September 9 anniversary of the nation's founding.
Officials from South Korea and Japan have confirmed that it was a nuclear test, North Korea's sixth. South Korea says it flew around 2,700 kilometres at a maximum altitude of about 550 kilometres.
Yesterday's missile landed nowhere near Guam, but firing a Hwasong-12 so soon after the threat may be a way for North Korea to show it could follow through if it chose to do so.
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