South Korea — North Korea carried out its sixth and most powerful nuclear test on Sunday, saying it had detonated a hydrogen bomb that could be mounted on an intercontinental ballistic missile. The test, which the North called a “complete success,” came hours after Pyongyang announced it had developed such a weapon. It was an extraordinary show of defiance by its leader, Kim Jong-un, against President Trump, who has threatened to bring “fire and fury” to the North if it continues to threaten the United States with nuclear missiles. But it was unclear whether the North had in fact tested a hydrogen bomb, a far more powerful weapon than the atomic bombs it has tested in the past. A seismic tremor detected at 12:36 p.m., emanating from the Punggye-ri underground nuclear test in northwestern North Korea, set off a scramble to determine whether the North had carried out another test. The South’s military soon confirmed that it had.
The United States Geological Survey estimated that the quake 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.
Just last week, North Korea fired a ballistic missile over Japan, sharply escalating tensions in the region. In July, Pyongyang launched an intercontinental ballistic missile capable of reaching the American mainland, and the North 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.
Japan’s foreign minister, Taro Kano, said Japan had requested an emergency meeting of the United Nations Security Council. President Moon Jae-in of South Korea and Prime Minister Shinzo Abe of Japan called emergency meetings of their national security council.
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, in which a small amount of thermonuclear fuel produced a slightly higher explosive yield but fell well short of a true hydrogen bomb.
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.
Another strategic consideration in responding to a nuclear blast is China, which for decades has been the North’s closest ally and its biggest trading partner. While China’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 appears to have lost 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 — China, Russia, India, Brazil 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.”