The nuclear crisis on the Korean Peninsula hit a crescendo Monday as South Korea’s military said it had uncovered signs the rival North was preparing for a fresh missile launch — possibly of a long-range weapon — and the Defense Ministry in Seoul announced it would push for the deployment of powerful U.S. strategic assets to the country.

The flurry of activity came just hours after Washington warned Pyongyang of a “massive military response” to “any threat” to the U.S. or its Asian allies in the wake of North Korea’s purported hydrogen bomb test.

The South’s spy agency said there is a chance that North Korea could fire an intercontinental ballistic missile on a standard trajectory into the northern Pacific Ocean, though it did not give a time frame, the South’s Yonhap news agency said.

It said the launch could come as early as Saturday, the anniversary of the founding of the Kim regime, or on Oct. 10, the establishment of the ruling Workers’ Party of Korea.

The Defense Ministry request for strategic assets, meanwhile, could include the deployment of a nuclear-powered aircraft carrier, heavy bombers and other powerful weapons to the peninsula.

“We will push for the option of deploying strategic assets such as the U.S. carrier strike group and strategic bombers after consultation with the U.S.,” the ministry said according to Yonhap.

Defense Minister Song Young-moo was quoted as saying that during recent talks with U.S. Defense Secretary James Mattis, he demanded the U.S. deploy the strategic assets to South Korea on a “regular” basis.

Song, Yonhap said, had cited local politicians’ calls for the redeployment of U.S. tactical nukes to South Korea in making the demand, but dismissed a report that he had actually demanded the redeployment of the U.S. nuclear arsenal.

The U.S. withdrew its tactical nuclear weapons from the peninsula in the early 1990s.

For its part, Seoul has appeared to take the reins amid the ongoing crisis, conducting a number of military drills in response to provocations from its neighbor. Drills on Monday included the firing of missiles into the sea, as part of a military exercise aimed at practicing for strikes on the North’s main Punggye-ri nuclear test site.

Hours ahead of those drills, Mattis said in Washington that any threat by Pyongyang to the United States, Japan or South Korea would result in a response both “effective and overwhelming.”

In a brief televised statement after a meeting with U.S. President Donald Trump and top national security advisers, Mattis, standing next to Joint Chiefs of Staff Chairman Gen. Joe Dunford, said that the international community was united on denuclearizing the Korean Peninsula and that Washington’s commitment to its allies remained unwavering.

“We made clear that we have the ability to defend ourselves and our allies — South Korea and Japan — from any attack,” Mattis said. “And our commitments among the allies are ironclad.”

Working to shore up Washington’s Asian alliances, Mattis also said that a wide range of military options had been discussed with Trump.

“We have many military options, and the president wanted to be briefed on each one of them,” Mattis said.

“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.

While those comments were largely seen as boilerplate U.S. remarks directed at the North, Daniel Pinkston, an East Asia expert at Troy University in Seoul, noted Mattis’ use of the terms “any threat,” calling this phrasing “a poor choice of words.”

“If we responded with military actions to threats, we’d be at war every day,” Pinkston said.

Mattis also singled out North Korea’s leader, warning him to listen carefully to the international outcry sparked by Pyongyang’s sixth and apparently most powerful nuclear test.

“Kim Jong Un should take heed (of) the United Nations Security Council’s unified voice — all members unanimously agreed on the threat North Korea poses, and they remain unanimous in their commitment to the denuclearization of the Korean Peninsula — because we are not looking to the total annihilation of a country — namely, North Korea,” Mattis said. “But, as I said, we have many options to do so.”

Mattis did not detail specific options, but Trump and his top advisers likely discussed everything from strategic asset deployments and overflights of bombers to plans for taking out North Korean missile sites and the country’s top leadership.

Andrew O’Neil, a professor of international relations at Griffith University in Australia, said that absent the deployment of tactical nukes, the U.S. and Seoul could also strengthen their conventional military presence in response to the North.

“We can probably expect a more aggressive flight pattern from U.S. and ROK aircraft near North Korean airspace — possibly even traversing North Korean territory — to drive home the point that Washington and Seoul are willing to go to war with a view to reinforcing deterrence,” O’Neil said, using the acronym for South Korea’s formal name, the Republic of Korea.

Such a move would be fraught with peril, O’Neil said, “but the potentially bigger risk is that North Korea feels emboldened to engage in coercion against U.S. allies” if Washington does not intervene.

“That’s when inadvertent escalation becomes a real danger,” he said.

Monday’s tide of announcements came after the North hailed as a “perfect success” its test Sunday of a “two-stage thermonuclear weapon,” or hydrogen bomb, capable of being loaded onto an ICBM.

The North called “the operation of the nuclear warhead … fully guaranteed,” according to a statement carried by state-run media. “It also marked a very significant occasion in attaining the final goal of completing the state nuclear force.”

There was no independent confirmation that the device detonated was in fact a hydrogen bomb, but the large resulting earthquake near the Punggye-ri site appeared to indicate a more powerful blast than previous explosions by the North.

Giving an idea of the blast’s power, Japan’s Meteorological Agency said Sunday that tremors caused by the test were “at least 10 times as powerful” as those generated by the atomic bomb Pyongyang detonated in September last year.

On Monday, South Korea’s Defense Minister said that the test had delivered an estimated explosive yield of 50 kilotons.

But NORSAR, a Norwegian earthquake monitoring agency, estimated the yield at 120 kilotons — a figure significantly higher than the 15-kiloton “Little Boy” bomb dropped on Hiroshima and the 20-kiloton “Fat Man” dropped on Nagasaki at the end of World War II.

It was not clear what information the varying estimates were based on, but experts said it would take time for scientists to reach a more narrow conclusion over the blast’s actual yield.

Meanwhile, criticism of the nuclear test spread across the globe as diplomats from Japan, South Korea, the U.S. and other countries scrambled to set up emergency meetings on the crisis. The United Nations Security Council was also scheduled to meet later Monday in New York.

In Tokyo on Monday morning, Foreign Minister Taro Kono held a 20-minute teleconference with U.S. Secretary of State Rex Tillerson, with Kono saying the two shared the recognition that the test posed an “unprecedentedly grave and imminent” security threat to the international community.

Tokyo and Washington, they agreed, would further bolster cooperation to “maximize pressure” on the regime by adopting new U.N. sanctions, Kono said. He declined, however, to elaborate on what sort of possible new sanctions they had discussed.

The foreign minister also held a stream of meetings with ambassadors from nine nonpermanent members of the U.N. Security Council.

In those meetings, Kono said he emphasized Japan’s position that the latest experiment was an “act of intolerable violence,” and that the global community must send an “explicitly strong message” to the regime at the emergency Security Council session.

Kono also met Evgeny Afanasiev, Russia’s ambassador to Japan. The two, he said, agreed to cooperate closely in dealing with the North issue and to pursue their ultimate goal of denuclearizing the Korean Peninsula.

After speaking with Kono, however, Afanasiev reiterated Russia’s long-held preference for dialogue over pressure.

“President (Vladimir) Putin said there is no other way but (to) resolve this issue through diplomatic, political means. Dialogue is needed,” Afanasiev told reporters, adding that pressure alone is “not enough, not working.” Abe and Putin are scheduled to meet in Vladivostok later this week.

Likewise, Kono urged Chinese Charge d’Affaires Lin Shaobin and British Ambassador to Japan Paul Madden to work toward adopting a “new, more powerful” set of U.N. sanctions against the North.

Kono quoted the Chinese representative as agreeing that “denuclearization of the Korean Peninsula” was the ultimate goal, adding that the Chinese official had even expressed mild dismay over the test having been conducted on the first day of an annual meeting of BRICS leaders in the southeastern Chinese city of Xiamen.

The test was also met Sunday with a stern rebuke from Prime Minister Shinzo Abe, who slammed it as “absolutely unacceptable” and vowed a tough response at the U.N.

On Sunday evening, Abe and Trump agreed in telephone talks — their second of the day and fourth since last Tuesday — to work to heap even more pressure on the North.

In his initial response to the North’s first nuclear test under his administration, Trump on Sunday appeared to invoke the specter of military action to rein in Pyongyang while also blasting Seoul’s calls for dialogue with their neighbor.

“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!” he wrote in a tweet.

The U.S. has about 28,000 troops stationed in South Korea, and is obliged by treaty to defend it in the event of war.

But even as Trump grappled with the issue of deterring the North, a former top adviser in the Obama administration said the biggest challenge in the wake of the Pyongyang’s nuclear and missile advances was not deterrence, but reassuring its Asian allies.

In a series of tweets, Colin Kahl, a former deputy assistant to President Barack Obama and national security adviser to Vice President Joe Biden, said that while extended deterrence theoretically helps reassure Japan and South Korea, “a lot of reassurance comes down to trust.”

“Our allies have to believe we would trade San Francisco for Seoul or Toledo for Tokyo if push comes to shove,” he wrote, a reference to potentially sacrificing an American city in order to protect one in Japan or South Korea after the North’s advances in long-range missile technology that experts say puts much of the United States within range.

But Kahl said Trump “has done the opposite” so far, including by delivering angry remarks last month that he would rain down “fire and fury” on Pyongyang if it endangered the United States.

“Undermining alliance solidarity at this moment is dumb & dangerous. It emboldens Pyongyang, increases the risk of NK miscalculation & potentially incentivizes the ROK & Japan to seek their own independent nuclear arsenals,” Kahl wrote.

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