SEOUL, South Korea -- Following U.S. warnings to North Korea of a "massive military response," South Korea on Monday fired missiles into the sea to simulate an attack on the North's main nuclear test site, a day after Pyongyang 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, although 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, advancement 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.

Chang Kyung-soo, an official with South Korea's Defense Ministry, told lawmakers that Seoul 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, President Donald Trump threatened to halt all trade with countries doing business with the North, a veiled warning to China, and faulted South Korea for what he called "talk of appeasement."

A 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" Pyongyang. 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 the North'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 Pyongyang'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.

The Russian leader was speaking at the meeting of the Brics group (Brazil, Russia, India, China and South Africa) in Xiamen, China.

Although he condemned the North's test as "provocative", Mr Putin said: "Sanctions of any kind would now be useless and ineffective.

"They'd rather eat grass than abandon their [nuclear weapons] programme unless they feel secure. And what can establish security? The restoration of international law. We should promote dialogue among all interested parties."

Citing a "humanitarian aspect", Mr Putin said millions of people would suffer under tougher measures, adding: "Sanctions have been exhausted."

On Monday, at the United Nations in New York, US envoy Nikki Haley argued that only the strongest sanctions would enable the problem to be resolved through diplomacy.

German Chancellor Angela Merkel backed that stance on Tuesday, saying more sanctions were urgently needed to counter the North's "flagrant breach of international conventions".

(CNN)US Defense Secretary James Mattis warned of "a massive military response" to any threat from North Korea against the United States or its allies in a statement outside the White House after a meeting with President Donald Trump, Vice President Mike Pence and top national security advisers Sunday.

Mattis said Trump wanted to be briefed on each of the "many military options" for dealing with the North Korean nuclear threat.

"Our commitment among the allies are ironclad," 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."

Mattis called on North Korean leader Kim Jong Un to "take heed" of the UN Security ?Council's unanimous position against North Korea's nuclear program and again stressed the US military's position.

"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," Mattis said.