SEOUL, South Korea — North Korea carried out its sixth nuclear test on Sunday, according to the South Korean military, an extraordinary show of defiance by North Korea’s leader, Kim Jong-un, against President Trump.

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 Defense Ministry estimated that the tremor had a magnitude of 5.7, revising an earlier estimate of 5.6. But the United States Geological Survey’s estimate was much higher, at 6.3.

The test came just days after North Korea fired a ballistic missile over Japan, sharply escalating tensions in the region. And just hours earlier, the North claimed that it had developed a hydrogen bomb that could be mounted on an intercontinental ballistic missile, though it offered no real evidence for that.

Mr. Trump has warned he would unleash “fire and fury” against North Korea if Pyongyang continued to threaten the United States with nuclear missiles.

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. 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.

Motoko Rich contributed reporting from Tokyo.

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