President Moon Jae-in of South Korea and Prime Minister Shinzo Abe of Japan called emergency meetings of their national security councils after the tremor was detected. “If North Korea has conducted a nuclear test, we can never accept that,” Mr. Abe told reporters.
Earlier in the day, Mr. Abe and Mr. Trump had spoken by telephone and resolved to put more pressure on North Korea.
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.
It was unclear whether such a weapon may have been tested on Sunday. Even South Korea’s estimate of 5.7 magnitude would make the blast considerably larger than the North’s previous one last September, which set off a tremor with an estimated magnitude of 5.2.
Mr. Trump’s aides have concluded that his options in responding to a North Korean nuclear blast 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.
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 timing of the test 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.”
Continue reading the main story