Though not yet confirmed, Pyongyang's apparent show of force was extraordinary — the hydrogen weapon is vastly more powerful than the atomic bombs dropped on the Japanese cities of Hiroshima and Nagasaki — and drew swift condemnation in capitals around the globe. Japanese Prime Minister Shinzo Abe called the action "absolutely unacceptable."
In Washington, Trump declared North Korea's latest provocation "very hostile and dangerous to the United States" and would not rule out a retaliatory strike. Asked as he left morning services at St. John's Church whether he was planning to attack North Korea, Trump told reporters, "We'll see."
Trump sought to assign responsibility for the unfolding crisis to North Korea's neighbors in the Asia-Pacific region, firing off a series of tweets that signaled rifts in U.S. economic and security partnerships that for years have helped isolate and contain North Korea.
It fell to Defense Secretary Jim Mattis to offer reassurances to the world that "the commitments among the allies are ironclad."
Trump, who has been weighing termination of a free-trade agreement with South Korea, scolded the longtime U.S. ally for not being tough enough in managing the northern threat. He tweeted, "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!"
South Korean President Moon Jae-in chose not to engage in an argument with Trump. He said his government is intent on achieving the peaceful denuclearization of the Korean Peninsula, in concert with "our allies."
The South Korean military conducted bombing drills at dawn Monday, practicing ballistic missile strikes on the North Korean nuclear test site at Punggye-ri.
Trump also said on Twitter that he was considering cutting off trade with any nation doing business with North Korea. China is by far the country's largest trading partner, but it also is the largest U.S. trading partner in terms of goods imported and exported. Such a move would amount to Trump's biggest trade salvo to date and would be nearly impossible to pull off without devastating the U.S. and global economies.
It would also drive up prices on many consumer goods. In 2016, U.S. companies exported $169.3 billion in goods to China and China exported $478.9 billion in goods to the United States.
Trump convened a Sunday afternoon White House meeting of his national security team, also attended by Vice President Pence. Mattis said that at the president's request they reviewed every military option and that Trump concluded the United States is prepared to defend itself and its allies.
"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," said Mattis, who was flanked by Gen. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff.
Mattis added, "We are not looking for the total annihilation of a country, namely North Korea, but as I said, we have many options to do so."
The U.N. Security Council, whose members voted unanimously last month to pass a package of sanctions against North Korea, called an emergency meeting for Monday morning at the urging of the United States, Japan, South Korea and France, according to Nikki Haley, the United States' U.N. ambassador.
Trump spoke Sunday by phone with Abe, and the two leaders confirmed the mutual defense commitments between the United States and Japan, according to the White House.
"President Trump reaffirmed the commitment of the United States to defending our homeland, territories, and allies using the full range of diplomatic, conventional, and nuclear capabilities at our disposal," read a statement from the office of White House press secretary Sarah Huckabee Sanders.
Trump's response to North Korea's weekend nuclear test — its sixth ever, but the first since Trump took office — was subdued relative to his bellicose war of words last month with that country's 33-year-old leader, Kim Jong Un.
In a pair of tweets issued Sunday morning, Trump wrote: "North Korea has conducted a major Nuclear Test. Their words and actions continue to be very hostile and dangerous to the United States . . . 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."
Nearly four weeks ago, Trump warned Kim that his continued nuclear provocations would be "met with fire and fury like the world has never seen."
Initially, North Korea seemed to back down from its threat of a nuclear strike in Guam, where many U.S. military personnel are stationed. Trump said of Kim at an Aug. 22 rally in Phoenix, "I respect the fact that, I believe, he is starting to respect us."
That assessment turned out to be premature.
"North Korea right now is the most dangerous place on the face of the planet," Sen. Ted Cruz (R-Tex.) said on ABC's "This Week." Cruz said of Kim, "He is radical, he is unpredictable, he is extreme, and he is getting more and more dangerous weapons."
Gen. Michael Hayden, a former director of the CIA and the National Security Agency, warned that Trump's tweets could foul up his otherwise respectable plan to get tough on North Korea.
"You gotta watch the tweets," Hayden said on CNN's "State of the Union." "Mr. President, this is not a manhood issue; this is a national security issue. Don't let your pride get in the way of wise policy here."
Treasury Secretary Steven Mnuchin, interviewed on "Fox News Sunday," said he intended to "draft a sanctions package to send to the president for his strong consideration that anybody who wants to do trade or business with them is prevented from doing trade or business with us."
"We are going to work with our allies, we'll work with China, but people need to cut off North Korea economically," Mnuchin added. "This is unacceptable behavior."
The tumult in the region comes amid escalating economic tensions with South Korea, a long-standing economic and diplomatic partner of the United States. Trump is considering withdrawing from a free-trade agreement with the country, in keeping with his campaign promise.
The move would end what Trump considers unfair trade competition from other countries. But the president's advisers have cautioned that a withdrawal from the agreement would strain ties with South Korea amid the mounting North Korea nuclear crisis.
Asked by Fox anchor Chris Wallace whether Trump would pull the United States out of the agreement, Mnuchin said, "The president has made clear that where we have trade deficits with countries, we're going to renegotiate those deals." He added that there have been "no decisions" yet with regard to the trade accord with South Korea.
Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.) said he spoke Sunday morning with White House Chief of Staff John F. Kelly about the situation.
"We stand ready to work with the administration to support a comprehensive strategy that not only places an emphasis on deterrence but also empowers our allies and partners in the region, who must do far more to confront this threat," Corker said in a statement.
Sen. Jeff Flake (R-Ariz.) acknowledged that "there are no good options" to manage the North Korea crisis but argued that "harsh rhetoric" does not appear to help slow Kim's nuclear program.
Flake said that ending the U.S.-South Korea trade agreement would be inadvisable.
"I don't think that that would be good in any circumstances," Flake said on CNN. "Now it's particularly troubling, given what South Korea is faced with. I think we need to do more trade, not less, and withdrawing from trade agreements is a very troubling sign."
Anna Fifield in Tokyo and Karoun Demirjian, Damian Paletta and Hamza Shaban in Washington contributed to this report.