Though the precise strength of the blast has yet to be determined, the artificial earthquake it caused was several times stronger than tremors generated by its previous tests. It reportedly shook buildings in China and in Russia.
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The test was carried out at 12:29 p.m. local time at the Punggye-ri site where North Korea has conducted nearly all of its past nuclear tests. Officials in Seoul put the magnitude at 5.7, while the U.S. Geological Survey said it was a magnitude 6.3.
North Korea's state-run television broadcast a special bulletin Sunday afternoon to announce the test. It said leader Kim Jong Un attended a meeting of the ruling party's presidium and signed the go-ahead order. Earlier in the day, the party's newspaper ran a front-page story showing photos of Kim examining what it said was a nuclear warhead being fitted onto the nose of an intercontinental ballistic missile.
North Korea in July test-launched two ICBMs that are believed to be capable of reaching the mainland United States. That would be a major step forward for Pyongyang, which says its missile development is part of a defensive effort to build a viable nuclear deterrent that can target U.S. cities.
The U.S. State Department had no immediate reaction.
China's foreign ministry said in a statement that the Chinese government has "expressed firm opposition and strong condemnation." It urged North Korea to "stop taking erroneous actions that deteriorate the situation."
South Korea held a National Security Council meeting chaired by President Moon Jae-in. National Security Director Chung Eui-yong said Moon will seek every available measure, including new U.N. sanctions or the deployment of more U.S. military assets, to further isolate Pyongyang.
Japan's Prime Minister Shinzo Abe called a test "absolutely unacceptable."
The nuclear test is the North's first since U.S. President Donald Trump assumed office in January. Trump has been talking tough with the North over its stepped-up missile tests, including a comment that Pyongyang would see fire, fury and power unlike any the world had ever witnessed if it continued even verbal threats.
The North claimed the device it tested was a thermonuclear weapon - commonly called an H-bomb. That could be hard to independently confirm. It said the underground test site did not leak radioactive materials, which would make such a determination even harder.
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North Korea conducted two nuclear tests last year, the last nearly a year ago, on the Sept. 9 anniversary of the nation's founding. It has been launching missiles at a record pace this year. It conducted its most provocative test yet last month, in response to ongoing U.S.-South Korea military exercises, when it fired a potentially nuclear-capable mid-range missile over northern Japan.
It said that launch was the "curtain-raiser" for more activity to come.
The photos released earlier Sunday showed Kim talking with his lieutenants as he observed a silver, peanut-shaped device that the state-run media said was a thermonuclear weapon designed to be mounted on the North's "Hwasong-14" ICBM.
The North claims the device was made domestically and has explosive power that can range from tens to hundreds of kilotons. Outside experts suggested the yield of the device tested Sunday might be in that ballpark, though closer to the lower range. For context, the bomb dropped on Hiroshima by the United States had a 15-kiloton yield.
North Korea's nuclear and missile program has made huge strides since Kim rose to power following his father's death in late 2011.
Its recent activity has been especially bold.
The North followed its two tests of Hwasong-14 ICBMs by announcing a plan to launch a salvo of intermediate range missiles toward the U.S. Pacific island territory of Guam. Kim Jong Un has reportedly signed off on the plan but is watching the moves by the U.S. before deciding when or whether to carry it out.
North Korea flew a Hwasong-12 over northern Japan last week, the first such overflight by a missile capable of carrying nukes, in a launch Kim described as a "meaningful prelude" to containing Guam. Guam is a major sore point for Pyongyang because it is a U.S. military hub and home to a squadron of B-1B bombers, which the North fears could be used to attack their country.
The U.S. on Thursday had sent the bombers and F-35 stealth fighters to the skies of South Korea in a show of force - and North Korea strongly protested.
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Nuclear tests are crucial to perfect sophisticated technologies and to demonstrate to the world that claims of nuclear prowess are not merely a bluff.
The White House said Trump spoke with Abe regarding "ongoing efforts to maximize pressure on North Korea." But the statement did not say whether the conversation came before or after the North's latest test.
The options they have to pressure Pyongyang would appear to be limited. Further economic and trade sanctions, increased diplomatic pressure and boosting military maneuvers or shows of force would likely all be on the table.
A long line of U.S. presidents has failed to check North Korea's persistent pursuit of missiles and nuclear weapons. Six-nation negotiations on dismantling North Korea's nuclear program in exchange for aid fell apart in early 2009.
Just before the test, according to state media, Kim and the other senior leaders at the party presidium meeting discussed "detailed ways and measures for containing the U.S. and other hostile forces' vicious moves for sanctions."
Kim, according to the statement carried by the state-run Korean Central News Agency, claimed all components of the device were domestically produced, which he said means the North can make "as many as it wants."
The two Koreas have shared the world's most heavily fortified border since their war in the early 1950s ended with an armistice, not a peace treaty.
About 28,500 American troops are deployed in South Korea as deterrence against North Korea.