South Korea's military fired a series of missiles into the Sea of Japan in an exercise Monday meant to simulate an attack on North Korea's nuclear test site.
The exercise came as the U.N. Security Council prepared to hold an emergency meeting later in the day to discuss North Korea's test of a nuclear weapon.
South Korean President Moon Jae-in and Japanese Prime Minister Shinzo Abe agreed in a phone call that North Korea should face harsh new sanctions, a South Korean presidential spokesman told reporters.
South Korea's defense ministry said Monday it had detected signs North Korea was preparing to test another ballistic missile. The ministry also announced plans to soon temporarily deploy four more launchers for the THAAD missile defense system.
China's foreign ministry said it lodged a protest with North Korea's embassy in Beijing over the nuclear test, and that North Korea is clear about China's commitment to the denuclearization of the Korean peninsula.
U.S. Defense Secretary Jim Mattis, echoing the warnings of President Donald Trump, said Sunday that North Korea can expect a “massive military response” if it threatens the United States, the U.S. territory of Guam or America's allies.
White House officials said the president emphasized the range of retaliatory measures available to the U.S., including nuclear weapons, in a conversation earlier with Japanese Prime Minister Abe: “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.”
WATCH: International response to North Korea
Mattis, Trump and the president's top advisers met at the White House about North Korea's announced hydrogen bomb test. The Pentagon chief came out to talk with reporters briefly afterwards to say the U.S. is not looking for the “total annihilation” of North Korea, but “we have many options to do so.”
North Korea's repeated provocative ballistic missile tests and now a sixth nuclear test seen as perhaps the first time Pyongyang has successfully detonated a thermonuclear device have presented Trump with this most critical geopolitical crisis of his young administration.
WATCH: Trump's response
“Secretary Mattis expressed the only viable option in his statement, which is a firm and clear deterrent policy toward North Korea," said Hoover Institution Fellow Michael Auslin.
However, Auslin told VOA, the goal of North Korea's denuclearization, which Mattis also repeated Sunday, is unrealizable.
“Continuing to insist on denuclearization means further rounds of negotiations, and the past quarter-century has shown that negotiations do not work,” he says. “The Trump administration has the opportunity to chart a new, more realistic course for U.S. policy, but not if it adopts the failed policies and goals of previous administrations.”
Other analysts and officials reacting to the extraordinarily stark remark from the Pentagon chief are hoping for diplomatic discussions instead of more tough military talk.
Mattis' “imprecision was counterproductive. Will there be a massive military response against any ‘threat’? This word choice was a blunder along the lines of the promise of ‘fire and fury’ against any North Korean threats,” said Frank Aum, a visiting scholar at the U.S.-Korea Institute.
Aum, the former senior adviser for North Korea at the Defense Department, told VOA: “It's telling that the defense secretary was the one who was addressing the press. We need to get away from a military-centric approach to the North Korea problem set and reinvigorate diplomacy.”
The president, leaving a church service near the White House earlier Sunday, said only, “We'll see” when a reporter asked if he was planning to order an attack on North Korea.
On Twitter, Trump said he is considering halting all trade with any country doing business with North Korea, raising immediate questions about what this could mean for U.S.-China commercial ties and the two countries' $650 billion in annual trade.
Any U.S. call for an economic boycott of countries doing business with North Korea would focus most sharply on China because Beijing is North Korea's sole major ally and its biggest trading partner.
Treasury Secretary Steve Mnuchin said he would prepare a new package of North Korea economic sanctions along these lines for consideration.
“We will work with our allies. We will work with China,” Mnuchin told a television interviewer (Fox News) Sunday. “But people need to cut off North Korea economically. This is unacceptable behavior.”
The North claimed its test of a hydrogen bomb small enough to be carried by an intercontinental ballistic missile was a “perfect success.”
One U.S. intelligence official says there is no reason to doubt North Korea's claim that the nuclear device it detonated underground Sunday was 10 times more powerful than its fifth nuclear test a year ago.
“We're highly confident this was a test of an advanced nuclear device and what we've seen so far is not inconsistent with North Korea's claims,” the intelligence official said.
North Korea test-fired two ICBMs in July that were believed to have a range long enough to reach the mainland United States.
Pyongyang says its missile development is a defensive effort to protect itself from U.S. attack.
Pyongyang and Washington have carried out a war of increasingly bellicose threats in recent weeks, with North Korea at one point saying it was planning to launch one or more test missiles toward the U.S. territory of Guam — evidently intending not to strike Guam, but to aim its rocket to splash down just outside territorial waters. Trump responded then that if Pyongyang attacked the United States or its allies, he would respond with “fire and fury like the world has never seen.”
United Nations Secretary-General Antonio Guterres condemned the nuclear test as “profoundly destabilizing for regional security.”
National Security Correspondent Jeff Seldin, Brian Padden in Seoul contributed to this report