WASHINGTON (REUTERS) - US Defence Secretary Jim Mattis issued a stark warning to North Korea on Wednesday (Aug 9), telling Pyongyang that it should stop any actions that would lead to the "end of its regime and the destruction of its people."

Mattis' words, some of the strongest he has issued on North Korea, followed incendiary comments from President Donald Trump who said on Tuesday that threats to the United States from Pyongyang would be met with "fire and fury."

Trump's unexpected remarks prompted North Korea to respond by saying it was considering plans for a missile strike on the US Pacific territory of Guam.

Mattis said in a statement that the United States and its allies would win any arms race or conflict with North Korea.

"The DPRK must choose to stop isolating itself and stand down its pursuit of nuclear weapons," Mattis said, using the acronym for North Korea's official name, the Democratic People's Republic of Korea.

"The DPRK should cease any consideration of actions that would lead to the end of its regime and the destruction of its people," he added.

On global markets, the strong rhetoric and sharp increase in tensions drove investors out of stocks and other risky assets on Wednesday and into textbook safe havens like gold and Treasuries.

The United States and South Korea remain technically still at war with North Korea after the 1950-53 Korean conflict ended with a truce, not a peace treaty.

Tension in the region has risen since North Korea carried out two nuclear bomb tests last year and two intercontinental ballistic missile tests in July. Trump has said he will not allow Pyongyang to develop a nuclear weapon capable of hitting the United States.

On Wednesday, Trump followed up his "fire and fury" warning with a boast about US nuclear capabilities.

"My first order as President was to renovate and modernise our nuclear arsenal. It is now far stronger and more powerful than ever before," Trump tweeted.

"Hopefully, we will never have to use this power, but there will never be a time that we are not the most powerful nation in the world!"

Trump's "fire and fury" remarks prompted warnings from US officials and analysts not to engage in rhetorical slanging matches with Pyongyang.

US Secretary of State Rex Tillerson sought to play down the rhetoric. Before Trump's tweets on the nuclear arsenal, Tillerson arrived in Guam on a previously scheduled visit after telling reporters he did not believe there was an imminent threat from North Korea and "Americans should sleep well at night."

With his "fire and fury" warning, Trump was "sending a strong message to North Korea in language that (North Korean leader) Kim Jong Un would understand, because he doesn't seem to understand diplomatic language," Tillerson said.

North Korea regularly threatens to destroy the United States.

GUAM THREAT

North Korea said it was "carefully examining" a plan to strike Guam, which is home to about 163,000 people and a US military base that includes a submarine squadron, an airbase and a Coast Guard group.

The plan would be put into practice at any moment, once Kim Jong Un made a decision, a Korean People's Army spokesman said.

Guam Governor Eddie Calvo dismissed the threat and said the island was prepared for "any eventuality" with strategically placed defences. He said he had been in touch with the White House and there was no change in the threat level.

North Korea, pursuing missile and nuclear weapons programmes in defiance of UN Security Council resolutions, accuses the Washington of devising a "preventive war" and has said any plans to execute this would be met with an "all-out war, wiping out all the strongholds of enemies, including the US mainland."

Washington has warned it is ready to use force if needed to stop North Korea's ballistic missile and nuclear programmes but that it prefers global diplomatic action, including sanctions.

The UN Security Council unanimously imposed new sanctions on North Korea on Saturday.

For the moment, USmilitary officials sought to play down the potential for military conflict. Three US officials, speaking on the condition of anonymity, said the United States had not moved additional assets into the region after North Korea's threats against Guam.

"Just because the rhetoric goes up, doesn't mean our posture changes," one official said.

"The only time our posture goes up is based on facts, not because of what Kim and Trump say to each other," the official added.

While Trump said the nuclear arsenal was more powerful than ever before, US officials say it takes decades to actually modernise nuclear weapons, a move already under way under President Barack Obama's administration, and there are treaties that regulate nuclear expansion. Trump signed an executive order after he took office in January to initiate a review of the country's nuclear policy and strategy.

'COMPLEX AND SENSITIVE'

A senior administration official who deals with the Korea issue said the "fire and fury" comment, which was Trump's strongest warning yet for North Korea and which he delivered to reporters in New Jersey, was "unplanned and spontaneous."

"There had not been any discussions about escalating the rhetoric in response to Kim's statements or about the possible effects of doing that," the official said.

The official added, however, "it is important for the North Koreans to understand that this country's strategic patience is exhausted and that our resolve to defend our allies, whatever is required to do that, is not."

In his comments on Tuesday, Trump said, "North Korea best not make any more threats to the United States. They will be met with fire and fury like the world has never seen."

Critics included fellow Republican John McCain, head of the Armed Forces Committee in the US Senate, who said Trump should tread cautiously. "You've got to be sure you can do what you say you're going to do," McCain said in a radio interview.

Steny Hoyer, the No. 2 Democrat in the US House of Representatives, said Trump's threat to North Korea "is reckless and shows a serious lack of judgment."

China, which is North Korea's closest ally despite its anger at Pyongyang's missile and nuclear programmes, described the situation as "complex and sensitive," and urged calm and a return to talks.

"China calls on all sides to uphold the main direction of a political resolution to the Korean peninsula nuclear issue, and avoid any words or actions that may intensify the problem and escalate the situation," the government said in a statement sent to Reuters, repeating its customary stance.

North Korea has made no secret of its plans to develop a nuclear-tipped missile able to strike the United States and has ignored all calls to halt its weapons programs. It says its ICBMs are a legitimate means of defense against perceived US hostility, including joint military drills with South Korea.

The South Korean capital, Seoul, is within range of massed North Korean rockets and artillery, which would be impossible to destroy in a first US strike. Tens of thousands of US troops remain stationed in South Korea and in nearby Japan.