AMERICA’S top diplomat Rex Tillerson has told his fellow citizens to “sleep well at night” and not worry about nuclear war with North Korea.

The comments from the US Secretary of State came as the regime in Pyongyang threatened to attack the US territory of Guam.

That was in response to US President Donald Trump promising to respond to nuclear threats from Kim Jong-un with “fire and fury like the world has never seen”.

According to the New York Times, Trump’s inflammatory remarks during what was supposed to be a meeting about the opioid crisis, were completely improvised.

Tillerson said the threats emanating in recent days from the North Korean government had forced Trump to respond in a language Jong-un would understand.

“What the President is doing is sending a strong message to North Korea in language that Kim Jong-un would understand, because he doesn’t seem to understand diplomatic language.”

Despite Tillerson’s attempts to calm fears after his boss’s remarks, Trump doubled down.

He replied to the new threat from the totalitarian government, warning that under his presidency, the US nuclear arsenal had become “far stronger and more powerful than ever before”.

The half-Scottish, real-estate tycoon turned leader of the free world, took to Twitter while at his golf course in New Jersey, claiming his order to”renovate and modernise” the US nuclear arsenal on the first day of his presidency meant the USA was number one.

“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,” he tweeted.

Though American media pointed out the modernisation of the US nuclear arsenal had started under Barack Obama and no substantial changes have been made since Trump took office.

Tillerson’s more moderated comments came as he spoke to reporters on board an official flight returning from Malaysia to Washington, stopping, coincidently, along the way in Guam.

“I think Americans should sleep well at night, have no concerns about this particular rhetoric of the last few days,” Tillerson said.

He added: “Nothing I have seen and nothing I know of would indicate that situation has dramatically changed in the last 24 hours.”

North Korea’s ballistic missile program has tested two intercontinental ballistic missiles in recent weeks, stoking fears among experts that Pyongyang’s nuclear ambitions are more advanced than previously thought.

The Washington Post quoted sources in American intelligence agencies saying North Korea had now miniaturised a warhead that could fit on top of one of its missiles.

In their annual threat assessment of their dictator neighbours, the Japanese government said they too now thought “it is possible that North Korea has already achieved the miniaturisation of nuclear weapons and has acquired nuclear warheads.”

It confirms what the North Koreans had claimed in December when they released a photograph of Jong-un posing with what looked a small disco ball, claiming it was a nuclear warhead.

The New York Times say the bomb, about two feet in diameter, would have a destructive yield equivalent to the atomic bombs dropped on Japan in World War II.

While North Korea has worked out how to fire a nuclear weapon, there is some doubt that it knows how to land one.

The Americans believes the Korean warheads would not survive the intense heat of re-entry from space. The 160,000 people living on Guam, who are technically American by birth, but who pay no taxes to the States, are hoping they won’t be the first ones to find out.

The tiny, strategically important, US territory in the western Pacific Ocean is 2200 miles southeast of North Korea, and 4000 miles to the east of Hawaii.

Around a third of the island is given over to US military, with two bases housing around 6000 troops.

A Korean people’s army spokesman said a plan would be put into practice as soon as the order to attack Guam was issued by the country’s leader, Kim Jong-un.

“The KPA strategic force is now carefully examining the operational plan for making an enveloping fire at the areas around Guam with medium- to long-range strategic ballistic rocket Hwasong-12 in order to contain the US major military bases on Guam, including the Anderson air force base,” the spokesman said.

The state-run KCNA news agency quoted a second army spokesman accusing Washington of devising a “preventive war”, adding that any attempt to attack the North would provoke “all-out war, wiping out all the strongholds of enemies, including the US mainland”. The US should cease its “reckless military provocation” against North Korea to avoid such a reaction, the spokesman added.

In a message to the public, the governor of Guam Eddie Baza Calvo said there was currently “no threat” to the island, but that they were “prepared for any eventuality”.

The UK Foreign Office said it would “continue to work with the US and our international partners to maintain pressure on North Korea”.

“We have been consistently clear and forthright in our condemnation of North Korea’s destabilising and illegal behaviour, including through support for UN Security Council resolutions to bring in sanctions that will limit North Korea’s ability to pursue its nuclear weapons programme,” a spokesman said.”

Tillerson had earlier said the only way out for the North Koreans would be through “talks”.