The North has tested a hydrogen bomb with "perfect success", a jubilant newsreader announced on state television, adding the device could be mounted on a missile.
The test was of a bomb with "unprecedently large power", she said, and "marked a very significant occasion in attaining the final goal of completing the state nuclear force".
The broadcaster showed an image of leader Kim Jong-Un's handwritten order for the test to be carried out at noon on September 3.
The announcement came after monitors measured a 6.3-magnitude tremor near the North's main testing site.
The blast yield from the nuclear test is estimated to be 100 kilotons, up to five times bigger than the bomb dropped on Nagasaki in 1945, and 10 times more powerful than the North's fifth atomic test.
Hours earlier, the North released images of Kim inspecting what it said was a miniaturised H-bomb that could be fitted onto an ICBM, at the Nuclear Weapons Institute.
Hydrogen bombs or H-bombs -- also known as thermonuclear devices -- are far more powerful than the relatively simple atomic weapons the North was believed to have tested so far.
Whatever the final figure for test's yield turned out to be, said Jeffrey Lewis of the armscontrolwonk website, it was "a staged thermonuclear weapon" which represents a significant advance in its weapons program.
Chinese monitors said they had detected a second quake shortly afterwards of 4.6 magnitude that could be due to a "collapse (cave in)", suggesting the rock over the underground blast had given way.
Pyongyang has long sought the means to deliver an atomic warhead to the United States, its sworn enemy, and the test will infuriate Washington, Tokyo, Seoul, Beijing and others.
Japanese Prime Minister Shinzo Abe said ahead of the announcement that a test would be "absolutely unacceptable".
South Korean President Moon Jae-In summoned the National Security Council for an emergency meeting and Seoul's military raised its alert level.
