The Trump administration was weighing all options Sunday night, including a retaliatory military strike, in response to North Korea’s test of a powerful hydrogen bomb that Pyongyang claimed could be mounted on an intercontinental ballistic missile capable of reaching the U.S. mainland.

The massive underground explosion early Sunday in a bunker south of North Korea’s border with China was Pyongyang’s sixth and by far most powerful nuclear detonation — so powerful that it registered as an earthquake.

Nerves were on edge around the world and in Washington, where President Trump called the test “very hostile and dangerous to the United States.”

A month after Mr. Trump made international headlines by claiming he would rain “fire and fury like the world has never seen” on North Korea if it continued with such provocations, the president’s response Sunday was notably more tempered — although he made it clear that his administration was seriously considering military retaliation.

“We’ll see,” Mr. Trump said when asked by a reporter whether he was planning to attack North Korea. He made the comment as he and first lady Melania Trump departed church in Washington late Sunday morning.

After the president met with national security advisers later in the day, Defense Secretary James Mattis issued a statement warning the North Koreans that the U.S. commitments to South Korea and Japan are ironclad.

“Any threat to the United States, or its territories, including Guam, or our allies will be met with a massive military response — a response both effective and overwhelming.”

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

The president did not reference military action in a series of tweets Sunday morning. Instead, he criticized South Korea for pursuing “appeasement” toward the North and lamented China’s failure — as North Korea’s neighbor and main economic backer — to contain Pyongyang.

“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,” the president tweeted.

He added, “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!”

While that hung in the backdrop throughout the day, Treasury Secretary Steven Mnuchin, a member of the administration’s National Security Council, suggested that the White House is more likely to respond economically than militarily — and to specifically increase pressure on China to cut all trade with Pyongyang.

The U.N. Security Council, with China’s blessing, adopted its harshest economic sanctions to date against North Korea last month, but analysts said Washington could and should be doing far more to target Chinese firms and banks with sanctions.

Mr. Mnuchin told Fox News that he would “draft a sanctions package to send it to the president for his strong consideration,” but he stopped short of explicitly indicating whether Chinese banks would be named.

“Anybody that wants to do trade or business with [North Korea] would be prevented from doing trade or business with us,” the Treasury secretary said. “We’ll work with China. China has a lot of trade with them. There’s a lot we can do to cut them off economically, much more than we’ve done already.”

50,000 tons of TNT

The detonation Sunday was the latest act of defiance by the regime of 33-year-old North Korean dictator Kim Jong-un, who has accelerated his nation’s pursuit of nuclear weapons and long-range missiles — in defiance of more than a decade of U.N. Security Council resolutions — since rising to power in Pyongyang after the 2011 death of his father and previous dictator Kim Jong-il.

The latest provocation, roughly a week after Pyongyang test-launched a ballistic missile that flew over the northern reaches of Japan, sent scientists scrambling to verify claims on North Korean state media that a hydrogen bomb had indeed been detonated underground on Sunday morning.

The U.S. Geological Survey said a 6.3-magnitude earthquake had been detected near North Korea’s known nuclear test site in Punggye-ri, in the county’s northeast region. The “artificial quake” had a depth of 14.3 miles and, according to South Korea’s state weather agency, was 9.8 times more powerful than a similar test carried out by the North in September 2016.

Last year’s blast registered as a 5.3-magnitude earthquake and, at the time, scientists described it as about as powerful as the nuclear bomb that the United States dropped on Hiroshima, Japan, in 1945, which was the equivalent of 15,000 tons of TNT.

A government official who spoke on the condition of anonymity with South Korea’s Yonhap News Agency on Sunday said the yield of Sunday’s test was estimated at about 50 kilotons, which is equal to that of 50,000 tons of TNT.

The detonation was felt in northern China in Yanji, near the North Korean border, according to local media, which cited the China Earthquake Administration as saying it had also detected a second tremor just after the main detonation. The Chinese agency termed the tremor a “collapse” and said it had 4.6 earthquake magnitude.

Chinese officials were reportedly also scrambling to conduct radiation testing along the border to determine more about the type of the explosion.

Nuclear weapons specialist Catherine Dill said Sunday that it was not clear exactly what nuclear weapon design had been tested. “But based on the seismic signature, the yield of this test definitely is an order of magnitude higher than the yields of the previous tests,” Ms. Dill told the BBC.

Current information did not definitively indicate that a thermonuclear weapon had been tested, “but it appears to be a likely possibility at this point,” she said, according to the news organization, which noted that hydrogen bombs are many times more powerful than an atomic bomb.

China reacts sharply

China and Russia, which is also seen as a tacit backer of North Korea despite supporting the recent U.N. sanctions against Pyongyang, both condemned the detonation but called on all sides to engage in dialogue.

The Chinese Foreign Ministry, however, issued a statement of rare and sharp criticism at North Korea, which is officially known as the Democratic People’s Republic of Korea, or DPRK.

“The DPRK has ignored the international community’s widespread opposition and conducted a nuclear test again. The Chinese government expresses resolute objection to and strong condemnation of it,” the ministry’s statement said, according to Beijing’s state news agency, Xinhua.

The harshest regional reactions came from U.S. allies in the Pacific, where more than 80,000 U.S. military personnel are stationed in South Korea and Japan, in addition to American troops and equipment on the island of Guam.

South Korean President Moon Jae-in, who came into office this year proposing expanded dialogue and a more conciliatory posture toward the North, called for the “strongest possible response” to Pyongyang’s latest provocation.

“President Moon Jae-in said the country will never allow North Korea to continue advancing its nuclear and missile technologies,” Mr. Moon’s key security adviser, Chung Eui-yong, said at a press briefing, according to Yonhap, which maintained that Seoul would push for the most powerful sanctions by the U.N. Security Council, which is set to meet Monday, to completely isolate Pyongyang.

That assertion came against the backdrop of Mr. Trump’s tweet expressing frustration with South Korea earlier in the day. It also came amid reports of potential friction between Washington and Seoul on an unrelated matter that has found the Trump White House weighing whether to withdraw from a free trade agreement with South Korea.

Several influential business groups warned the Trump administration on Sunday against a pullout from a free trade deal known as KORUS, the prospect of which has also alarmed lawmakers and governors of states wary of losing jobs.

Call for more sanctions

U.S. lawmakers on both sides of the aisle issued statements calling on the administration to move quickly toward significantly ramping up economic sanctions against North Korea that also would target Chinese entities — whether Beijing likes it or not.

“Years of failed negotiations have left big holes in the sanctions on this rogue regime,” said House Foreign Affairs Committee Chairman Edward R. Royce, California Republican.

“Working with our allies, especially South Korea and Japan, we must apply maximum financial and diplomatic pressure,” Mr. Royce said in a statement. “That includes targeting more Chinese banks that do business with North Korea — with or without Beijing’s cooperation.”

Rep. Adam B. Schiff, California Democrat and the ranking member of the House Permanent Select Committee on Intelligence, told CNN that the latest bomb test “certainly underscores the heightened importance of getting China to work with us much more aggressively to cut off trade to North Korea.”

He added that “we’re going to have to start imposing secondary sanctions — that is, sanctions on other countries doing business with North Korea.”

While Sen. Ted Cruz, Texas Republican, told ABC News that the Kim regime in Pyongyang will “only understand and respect strength,” he too stopped short of advocating retaliatory military strikes because “no rational person wants to see a military conflict with North Korea.”

“With the nuclear weapons there, [in] almost any scenario, you’re looking at tens or thousands or hundreds of thousands of casualties in a matter of days,” said Mr. Cruz, who argued that the best way forward would be to expand U.S. missile defense systems in the region while also dramatically increasing economic sanctions against Pyongyang.

Sen. Edward J. Markey, Massachusetts Democrat and the ranking member of the Senate Foreign Relations East Asia subcommittee, said a combination of deeper sanctions and shuttle diplomacy to bring China, Russia and others toward a more unified front against Pyongyang is what’s needed.

The goal, he said, would be to resume long-stalled diplomatic negotiations with the Kim regime.

“We must exhaust every available diplomatic option to de-escalate tensions and negotiate an end to the North Korea nuclear threat before it erupts into a catastrophic war,” Mr. Markey said in a statement. “The international community must apply the kind of crushing economic pressure required to bring North Korea back to the negotiating table, including starving the country of oil and other economic supports that allow it to continue its missile programs.

“But we must also be willing to take measures to assure North Korea that our military forces are in the region only to deter and defend against aggression, not to attack or depose the North Korean regime,” he said. “As soon as North Korea agrees to a freeze of its missile and nuclear testing, we and our South Korean allies should continue direct negotiations with Pyongyang.”

• S.A. Miller and Dave Boyer contributed to this report.

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