SEOUL, SOUTH KOREA—North Korea’s detonation of a sixth nuclear bomb Sunday prompted the Trump administration to warn that even the threat to use such a weapon against the United States and its allies “will be met with a massive military response.”

The test, and the White House rebuke, immediately raised new questions about the president’s North Korea strategy and opened a new rift with a major U.S. ally, South Korea, which Trump criticized for its “talk of appeasement” with the North.

The underground blast — deemed a “perfect success” by the North Korean regime — was by far the country’s most powerful to date. Though it was far from clear that the North had set off a hydrogen bomb, as it claimed, the explosion caused tremors that were felt in South Korea and China. Experts estimated that the blast was four to 16 times more powerful than any the North had set off before, with far more destructive power than the bombs dropped on the Japanese cities of Hiroshima and Nagasaki during the Second World War.

Yet after a day of meetings in the Situation Room involving Trump and his advisers, two phone calls between the president and Prime Minister Shinzo Abe of Japan, and even demands from some liberal Democrats to cut off North Korea’s energy supplies, Trump’s aides conceded that they faced a familiar conundrum.

While the Pentagon has worked up a series of military options for targeted strikes at North Korea’s nuclear and missile sites, Trump was told that there is no assurance that the U.S. could destroy them all in a lightning strike, according to officials with knowledge of the exchange. Cyberstrikes, which president Barack Obama ordered against the North’s missile program, have also been judged ineffective.

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Trump hinted at one extreme alternative: in a Twitter post just before he met his generals, he said that “the United States is considering, in addition to other options, stopping all trade with any country doing business with North Korea.” Taken literally, such a policy would be tantamount to demanding a stoppage of any Chinese oil to North Korea, essentially an attempt to freeze out the country this winter and bring whatever industry it has to a halt.

The Chinese would almost certainly balk; they have never been willing to take steps that might lead to the collapse of the North Korean regime, no matter how dangerous its behaviour, for fear that South Korean and U.S. troops would occupy the country and move directly to the Chinese border.

Beyond that, the economic disruption of ending all trade with China would be so huge inside the U.S. that Trump’s aides declined Sunday to discuss the implications.

After meeting with Trump, Defence Secretary Jim Mattis emerged to warn North Korea that “any threat to the United States or its territory, including Guam or our allies, will be met with a massive military response.” But Mattis, in a terse statement delivered on the White House driveway with the chairman of the Joint Chiefs of Staff, Gen. Joseph Dunford Jr., also offered a word of reassurance to the North’s reclusive leader, Kim Jong Un.

“We are not looking to the total annihilation of a country, namely North Korea,” he said. “But as I said, we have many options to do so.”

There was no public discussion of pursuing a diplomatic opening to the North. Trump and Secretary of State Rex Tillerson raised such a possibility two weeks ago, after a brief lull in North Korea’s testing. That statement turned out to be optimistic at best. The North has shown no interest in engaging with the U.S. unless the Americans end their military presence in South Korea.

To the contrary, the North Korean leader has tried to portray his nuclear program as unstoppable and non-negotiable, posing by a picture of what the North’s official news agency on Sunday called a hydrogen bomb that could be fitted into the nose cone of the ICBMs tested last month. Experts warned that the weapon, while shaped like a hydrogen bomb, could well have been a mock-up or decoy, one of the many steps the North takes to make it appear more powerful than it truly is.

On Monday, South Korea’s army fired short-range ballistic missiles off its east coast in a simulated attack on North Korea’s nuclear test site, its military said in a statement. F-15K fighter jets also joined in the show of force, firing air-to-land missiles, it said.

Only hours earlier, Trump reacted to the North Korean test by lashing out at South Korea.

“North Korea is a rogue nation which has become a great threat and embarrassment to China, which is trying to help but with little success,” he said. “South Korea is finding, as I have told them, that their talk of appeasement with North Korea will not work, they only understand one thing!”

Trump appeared to be referring to the offers by South Korea’s new president, Moon Jae-in, to enter into some kind of negotiations with the North that might lead to a renewal of the “Sunshine Policy,’’ an effort by some of his predecessors to lure the North into disarmament with economic engagement. Those efforts failed.

Moon said recently that he had obtained a promise from Washington that the U.S. would never take military action without Seoul’s approval — a commitment the Trump administration has never confirmed.

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Trump’s undisguised swipe at the South for “appeasement” was certain to exacerbate fears that the U.S. might put it in danger. And it came only a day after Trump threatened a new rift in relations with suggestions that the U.S. might withdraw from a trade deal with South Korea — one that was intended to bolster the alliance.

In response to Trump’s criticism, Moon’s office said it was working closely with Washington to exert “maximum sanctions and pressure.” But it also reiterated that the allies shared the understanding that the goal of these sanctions and pressure was to bring North Korea back to the negotiating table.

“We have experienced an internecine war and can never tolerate another catastrophic war on this land,” Moon’s office said in a statement. “We will not give up our goal of working together with allies to seek a peaceful denuclearization of the Korean Peninsula.”

While Washington and Seoul argue over the threat of military force, Kim seems determined to forge ahead. He has conducted more than 80 missile tests since taking over the country. And four of the six nuclear tests have been on his watch.

This was the biggest, by far. The U.S. 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 Defence 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 South’s National Fire Agency, which operates an emergency hotline, said it had received 31 calls about buildings and the ground shaking, the first time that South Koreans had reported tremors after a North Korean nuclear detonation.

The blast was so powerful that the first tremor was followed by a second, weaker one minutes later, which the U.S. Geological Survey called a “collapse,” probably a cave-in at the North’s underground test site.

Condemnation of the test came from around the world.

China, the North’s main ally and biggest trading partner, expressed “strong condemnation” of the test, according to Xinhua, the state news agency, but suggested no new action. Its leaders feel as stymied as their U.S. counterparts, according to many China experts.

Japan requested an emergency meeting of the United Nations Security Council, as it did earlier in the week after a missile test over Hokkaido, its northernmost island.

In Europe, the Russian Foreign Ministry said that North Korea “deserves absolute condemnation,” and a joint statement from Chancellor Angela Merkel of Germany and President Emmanuel Macron of France said “the most recent provocation from Pyongyang reaches a new dimension.”

The International Atomic Energy Agency said the test amounted to a “complete disregard of the repeated demands of the international community.”

The timing of the test Sunday was almost certainly no coincidence: it came during the American Labour Day weekend, and the anniversary of the founding of the North Korean government is next Saturday.

In the coming days, the government is expected to organize huge rallies to celebrate the bomb test and 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.

But it also exaggerates its power. After 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 that used tritium, a common enhancement technique that produces a higher explosive yield.

Analysts noted that the device in the photo that the North released 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,” 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.”

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