Pyongyang's latest test has been swiftly condemned by countries including Russia, China, South Korea and Japan
North Korea has detonated a hydrogen bomb with "perfect success", the secretive country's state media has announced.
It added that the device was designed to be mounted on its newly developed intercontinental ballistic missile.
Earlier, Japan confirmed its near neighbour had conducted a sixth nuclear test.
Japan's meteorological agency said the resulting tremors were at least 10 times as powerful as North Korea's previous nuclear test, last September.
Experts estimated that blast to have been around 10 kilotons.
Donald Trump took to Twitter to comment on the blast, saying the Pyongyang's "words and actions continue to be very hostile and dangerous":
..North Korea is a rogue nation which has become a great threat and embarrassment to China, which is trying to help but with little success.— Donald J. Trump (@realDonaldTrump) September 3, 2017
South Korea is finding, as I have told them, that their talk of appeasement with North Korea will not work, they only understand one thing!— Donald J. Trump (@realDonaldTrump) September 3, 2017
Japan's foreign minister, Taro Kono, described the new explosion as "extremely unforgivable".
The Tokyo government has registered a protest with the North Korean embassy in Beijing, he said.
Hours earlier, Japanese Prime Minister Shinzo Abe spoke to President Trump on the phone about the "escalating" situation.
South Korea's president, Moon Jae-in, has called for the "strongest" response, while Russia's foreign ministry said the test was "another instance of disregard for requirements under UN Security Council resolutions and international law".
State media in China also reported that the foreign ministry there had condemned the test, which is said to have taken place near the China-North Korea border:
The US Geological Survey said a magnitude 6.3 tremor struck North Korea's northeast, near its weapons test site.
It comes just hours after Pyongyang claimed to have developed a hydrogen bomb that could be loaded into a long-range missile.
The state-run Korean Central News Agency released pictures of Kim Jong Un visiting the country's Nuclear Weapons Institute in Pyongyang.
Dressed in a black suit and flanked by his lieutenants, the country's leader was seen examining a metal casing with two bulges that was apparently the purported weapon.
State media quoted Mr Kim as saying it was a "thermonuclear weapon with super explosive power" and "all components of the H-bomb were 100% domestically made".
It claimed the hydrogen bomb's power is adjustable to hundreds of kilotons and can be detonated at high altitudes.
The latest test is likely to again increase tensions between the US and North Korea.
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.
Experts are sceptical about the claim that Pyongyang has mastered hydrogen technology, but it is almost impossible to independently confirm statements about its highly secret weapons programme.
Melissa Hanham, of the Middlebury Institute for International Studies in California, said the images released by the North could not be proved real.
"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.
"It doesn't need to be shaped like that on the outside, but they threw in a diagram, just so we would get the message.
"The bottom line is that they probably are going to do a thermonuclear test in the future. We won't know if it's this object though."
The developments are raising already high tensions on the Korean Peninsula and in Washington that the North is getting closer to its goal of an arsenal of intercontinental ballistic missiles (ICBMs) that can reach the US mainland.
Last month, US media reported that US intelligence officials had concluded Pyongyang had successfully miniaturised a nuclear weapon.
On Tuesday, the North fired a mid-range ballistic missile that flew over Japan - a test considered one of the most provocative ever from the reclusive state.