The United States Geological Survey estimated that the tremor set off by the blast, detected at 12:36 p.m. at the Punggye-ri underground test site in northwestern North Korea, had a magnitude of 6.3.
The South Korean Defense Ministry’s estimate was much lower, at 5.7, but even that would mean a blast “five to six times” as powerful as the North’s last nuclear test, a year ago, said Lee Mi-sun, a senior analyst at the South Korean Meteorological Administration.
The blast was so powerful that the first tremor was followed by a second, weaker one minutes later, which the United States Geological Survey called a “collapse.” The second tremor was detected in China but not in South Korea; officials in the South said that would be consistent with a cave-in at the North’s underground test site.
President Moon Jae-in of South Korea, a proponent of dialogue with North Korea, called the test “utterly disappointing and infuriating.” China, the North’s main ally and biggest trading partner, expressed “strong condemnation” of the test, according to Xinhua, the state news agency. Japan requested an emergency meeting of the United Nations Security Council.
The Russian Foreign Ministry said that North Korea “deserves the strongest condemnation,” The Associated Press reported, and the International Atomic Energy Agency said the test amounted to a “complete disregard of the repeated demands of the international community.”
Earlier in the day, Prime Minister Shinzo Abe of Japan and Mr. Trump had spoken by telephone and resolved to put more pressure on North Korea.
Just last week, North Korea fired a ballistic missile over Japan, sharply escalating tensions in the region. Pyongyang recently launched an intercontinental ballistic missile capable of reaching the American mainland, and it responded to Mr. Trump’s “fire and fury” rhetoric by threatening to fire missiles into waters around Guam, a United States territory that is home to military bases.
The timing of the test on Sunday was almost certainly no coincidence: It came during the American Labor Day weekend, and the anniversary of the founding of the North Korean government is Saturday.
The North has often tried to catch its enemies in Washington off guard by conducting major weapons tests around American holidays — Mr. Kim called his country’s first ICBM test, conducted on July 4, a “gift package for the Yankees” — or timed them to coincide with its own holidays for domestic propaganda uses.
On Sunday, North Korea gave its people and the outside world notice of what was to come, and it also displayed Mr. Kim’s handwritten order to conduct the test. In the coming days, the government is expected to organize huge rallies to celebrate the bomb test and Mr. Kim’s leadership.
“Pyongyang has a playbook of strategic provocations, throws off its adversaries through graduated escalation, and seeks maximum political impact by conducting weapons tests on major holidays,” said Lee Sung-yoon, a Korea expert at the Fletcher School of Law and Diplomacy at Tufts University.
North Korea has conducted a series of nuclear and ballistic missile tests since 2006. Its previous nuclear tests have produced increasingly larger blasts. The last test, in September 2016, yielded one about as powerful as the bomb the United States dropped on Hiroshima in 1945.
In its fourth nuclear test, in January 2016, North Korea claimed to have used a hydrogen bomb. Other countries dismissed the claim for lack of evidence, but experts have said that the North may have tested a “boosted” atomic bomb by using tritium, a common enhancement technique that produces a higher explosive yield.
Hours before the tremor was detected on Sunday, North Korea’s state news agency said the country had developed a hydrogen bomb that could be mounted on an intercontinental ballistic missile. The report offered no evidence for the claim, other than photos of Kim Jong-un, the country’s leader, inspecting what it said was the weapon.
Kim Dong-yub, a defense analyst at the Institute for Far Eastern Studies at Kyungnam University in Seoul, said he believed the device that North Korea detonated on Sunday was a “boosted” atomic bomb. He said the estimated explosive yield of 60 to 80 kilotons was too low for a bona fide hydrogen bomb, which can pack more than 1,000 times the destructive power of an ordinary nuclear weapon.
Analysts noted that the device in the photo that the North released on Sunday — whether real or a mock-up — was shaped like a two-stage thermonuclear device. David Albright, president of the Washington-based Institute for Science and International Security, said he doubted the device was real, but he said there was strong evidence that the North had been working on thermonuclear weapons.
“The size of the seismic signal of the recent test suggests a significantly higher explosive yield than the fifth test,” Mr. Albright said. “Getting this high of a yield would likely require thermonuclear material in the device.”
But he said he was “skeptical that this design has been miniaturized to fit reliably on a ballistic missile.”
Mr. Trump’s aides have concluded that his options in responding to the North Korean threat are limited. A strike on the North’s main nuclear and missile sites faces the same challenge it always has: the North’s ability to retaliate against Seoul, the South’s capital, which is within range of its artillery.
So for now, Mr. Trump has turned to the same strategy his predecessors have tried: increasing economic pressure and threatening military force, though Mr. Trump has used more provocative rhetoric about a potential military response than his predecessors did.
Another strategic consideration in responding to a nuclear blast is China. While the country’s president, Xi Jinping, fears that a collapse in North Korea could lead to a wave of hungry refugees and a scramble for North Korea’s territory and nuclear weapons, he has shown signs of losing patience with Mr. Kim, recently agreeing to stronger United Nations sanctions against Pyongyang.
The test’s timing was a major embarrassment for Mr. Xi, who on Sunday was hosting a summit meeting of the so-called Brics countries — Brazil, Russia, India, China and South Africa. Cheng Xiaohe, a North Korea expert at Renmin University in Beijing, said the timing of the test — on the day of the summit meeting’s opening ceremony, in the Chinese city of Xiamen — appeared to be deliberate.
“This will test whether China is prepared to go ahead with more radical actions like cutting off oil supplies to North Korea,” Mr. Cheng said.
Peter Hayes, director of Nautilus, a United States-based research institute specializing in North Korea, said the test seemed intended to jolt Mr. Xi, and to convince him that he needed to persuade the United States to talk to North Korea.
“It’s aimed more at Xi than Trump,” Mr. Hayes said. “Kim Jong-un doesn’t have the leverage to get Washington to talk. Xi has real power to affect the calculations in Washington. He’s putting pressure on China to say to Trump, you have to sit down with Kim Jong-un.”