SEOUL, South Korea >> Following U.S. warnings to North Korea of a “massive military response,” South Korea fired missiles into the sea to simulate an attack on the North’s main nuclear test site today, a day after North Korea detonated its largest-ever nuclear test explosion.

South Korea’s Defense Ministry also said Monday that North Korea appeared to be planning a future missile launch, possibly of an ICBM, to show off its claimed ability to target the United States with nuclear weapons, though it was unclear when this might happen.

The heated words from the United States and the military maneuvers in South Korea are becoming familiar responses to North Korea’s rapid, as-yet unchecked pursuit of a viable arsenal of nuclear-tipped missiles that can strike the United States. The most recent, and perhaps most dramatic, advance came Sunday in an underground test of what leader Kim Jong Un’s government claimed was a hydrogen bomb, the North’s sixth nuclear test since 2006.

The United Nations Security Council held its second emergency meeting about North Korea in a week on Monday, with U.N. political chief Jeffrey Feltman calling the test a “dangerous provocation” that was profoundly destabilizing for the region and the international community.

In Seoul, Chang Kyung-soo, an official with South Korea’s Defense Ministry, told lawmakers on Monday that it was seeing preparations in the North for an ICBM test but didn’t provide details about how officials had reached that assessment. Chang also said the yield from the latest nuclear detonation appeared to be about 50 kilotons, which would mark a “significant increase” from North Korea’s past nuclear tests.

In a series of tweets, U.S. President Donald Trump threatened to halt all trade with countries doing business with North Korea, a warning to China, and faulted South Korea for what he called “talk of appeasement.”

In response, Chinese Foreign Ministry spokesman Geng Shuang, told reporters in Beijing on Monday that China regarded as “unacceptable a situation in which on the one hand we work to resolve this issue peacefully but on the other hand our own interests are subject to sanctions and jeopardized. This is neither objective nor fair.”

South Korea’s military said its live-fire exercise was meant to “strongly warn” North Korea. The drill involved F-15 fighter jets and the country’s land-based “Hyunmoo” ballistic missiles firing into the Sea of Japan.

The target was set considering the distance to North Korea’s test site and the exercise was aimed at practicing precision strikes and cutting off reinforcements, Seoul’s Joint Chiefs of Staff said.

Each new North Korean missile and nuclear test gives the country’s scientists invaluable information that allows big jumps in capability. North Korea is thought to have a growing arsenal of nuclear bombs and has spent decades trying to perfect a multistage, long-range missile to eventually carry smaller versions of those bombs.

Both diplomacy and severe sanctions have failed to check the North’s decades-long march to nuclear mastery.

In Washington, Trump, asked by a reporter if he would attack North Korea, said, “We’ll see.” No U.S. military action appeared imminent, and the immediate focus appeared to be on ratcheting up economic penalties, which have had little effect thus far.

In brief remarks after a White House meeting with Trump and other national security officials, Defense Secretary Jim Mattis told reporters that America does not seek the “total annihilation” of North Korea, but then added somberly, “We have many options to do so.”

Mattis said the U.S. will answer any threat from the North with a “massive military response — a response both effective and overwhelming.”

Mattis also said the international community is unified in demanding the denuclearization of the Korean Peninsula and that Kim should know Washington’s commitment to Japan and South Korea is unshakeable.

The precise strength of North Korea’s underground nuclear explosion has yet to be determined. South Korea’s weather agency said the artificial earthquake caused by the explosion was five times to six times stronger than tremors generated by the North’s previous five tests.

Sunday’s detonation builds on recent North Korean advances that include test launches in July of two ICBMs. The North says its missile development is part of a defensive effort to build a viable nuclear deterrent that can target U.S. cities.

North Korea has made a stunning jump in progress in its nuclear and missile programs since Kim rose to power following his father’s death in late 2011. The North followed its two tests of Hwasong-14 ICBMs, which, when perfected, could target large parts of the United States, by threatening to launch a salvo of Hwasong-12 intermediate range missiles toward the U.S. Pacific island territory of Guam in August.

It flew a Hwasong-12 over northern Japan last week, the first such overflight by a missile potentially capable of carrying nuclear weapons, in a launch Kim described as a “meaningful prelude” to containing Guam, the home of major U.S. military facilities, and vowed to launch more ballistic missile tests targeting the Pacific.

Ahead of the North’s test, photos released by the North Korean government showed Kim talking with his lieutenants as he observed a silver, peanut-shaped device that was purportedly a thermonuclear weapon destined for an ICBM. The images were taken without outside journalists present and could not be independently verified. What appeared to be the nose cone of a missile could also be seen in one photo, and another showed a diagram on the wall behind Kim of a bomb mounted inside a cone.

The Arms Control Association in the United States said the explosion appeared to produce a yield in excess of 100 kilotons of TNT equivalent, which it said strongly suggests North Korea tested a high-yield but compact nuclear weapon that could be launched on a missile of intermediate or intercontinental range.

Beyond the science of the blast, North Korea’s accelerating push to field a nuclear weapon that can target all of the United States is creating political complications for the U.S. as it seeks to balance resolve with reassurance to allies that Washington will uphold its decades-long commitment to deter nuclear attack on South Korea and Japan.

That is why some questioned Trump’s jab at South Korea. He tweeted that Seoul is finding that its “talk of appeasement” will not work. The North Koreans, he added, “only understand one thing,” implying military force might be required. 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.

Trump also suggested putting more pressure on China, North Korea’s patron for many decades and a vital U.S. trading partner, in hopes of persuading Beijing to exert more effective leverage on its neighbor. Trump tweeted that the U.S. is considering “stopping all trade with any country doing business with North Korea.” Such a halt would be radical. The U.S. imports about $40 billion in goods a month from China, North Korea’s main commercial partner.

Experts have questioned whether North Korea has gone too far down the nuclear road to continue pushing for a denuclearization of the Korean Peninsula, an Obama administration policy goal still embraced by Trump’s White House.

“Denuclearization is not a viable U.S. policy goal,” said Richard Fontaine, president of the Center for a New American Security, but neither should the U.S. accept North Korea as a nuclear power, he said. “We should keep denuclearization as a long-term aspiration, but recognize privately that it’s unachievable anytime soon.”