Washington: As they confront the prospect of a nuclear attack on the US mainland by North Korea, Americans are more convinced by the seriousness of the threat than they are confident that their president is best suited to handle it.

In a poll conducted for the Chicago Council on Global Affairs, three-quarters of respondents rated the threat as critical – and more than 60 per cent said they would support sending US troops if North Korea was to invade South Korea.

But in a poll for CBS News, 61 per cent said they were "uneasy" about Donald Trump's capacity to deal with the crisis. Only 35 per cent were "confident".

It remains to be seen how those numbers might shift as the country digests Trump's break from a "working vacation" at his New Jersey golf club on Tuesday, to address the dictator Kim Jong Un in the kind of rhetoric expected more from Pyongyang than from Washington.

The address was prompted by a US intelligence conclusion that North Korea may have perfected a miniature nuclear warhead capable of being fitted to a missile that could reach the US.

Trump warned: "North Korea best not make any more threats to the United States [or] they'll be met with the fire and fury and frankly power, the likes of which this world has never seen before."

Recall, this is the man who pilloried his predecessor for imposing a red line on Syrian dictator Bashar al-Assad over chemical weapons attacks on his own people – and when Assad crossed the line, Barack Obama did nothing.

On Tuesday, Trump drew his own impossible red line and promised "fire and fury".  This, though, is not for a consequential act like another missile test, but for another nonsense "threat" of the kind the North's cliche-factory PR machine grinds out on a daily basis.

This is not to say it's not a grave crisis. The US Defence Intelligence Agency's alarming conclusion on further success by the North's run-away nuclear program coincides with Pyongyang thumbing its nose at the latest round of UN sanctions.

At this week's ASEAN summit in Manila, the North's Foreign Minister Ri Yong Ho said Kim would not surrender his "strategic [nuclear] option" in the face of a threatened US attack.

"We will, under no circumstances, put the nukes and ballistic rockets on the negotiating table," he said while provocatively claiming that all of the mainland US was now within range of the North's missiles.

Ri's use of the term "strategic option" is a small window into the logic of a besieged rogue nation. Looking at the fate of Libya's Muammar Gaddafi and Iraq's Saddam Hussein, Pyongyang figures that a headlong race to a demonstrable nuclear capacity is its best defense.

The official Korean Central News Agency said as much on Monday. It released a statement declaring that the "fear" with which the US and the rest of the world had responded to just two North Korean missile tests had made Pyongyang "more determined that this is the only way we can survive, the only path we can take."

And while many in the outside world assume Kim to be crazy, others disagree.

Siegfried Hecker, director emeritus of the Los Alamos National Laboratory, who was the last known American official to inspect the North's nuclear facilities, told reporters: "Some like to depict Kim as being crazy — a madman — and that makes the public believe that the guy is undeterrable. He's not crazy and he's not suicidal. And he's not even unpredictable".

The Defence Intelligence Agency's finding coincides with a similar conclusion by the Japanese Ministry of Defence. But perhaps Pyongyang's refusal to be cowed, even as China and Russia backed the latest round of sanctions, is more convincing proof that indeed the North has arrived, or is on the threshold, of fully-fledged membership of the global nuclear club.

Swanning around the ASEAN gathering, Ri didn't seem like a man with his back to the wall. And his swift rejection of a US proposal for talks that would include Beijing and Moscow perhaps spoke to his confidence in Pyongyang's claimed or suspected nuclear success. On the other hand, it may be a dangerous dose of the Comical Ali's, as in Saddam's hapless information minister who denied that Baghdad had fallen even as the TV's showed US-led forces in the Iraqi capital in 2003.

If the US and other intelligence agencies are correct, North Korea has hit a nuclear quenelle – it has devised a missile that can strike anywhere in the US mainland and is able to arm it with a nuclear warhead. Similarly, while the North was previously thought to have just a few nuclear warheads, the US agencies now count them in the dozens.

That seeming success calls up another Trump red line. In January, when Pyongyang claimed it was finessing a missile capable of striking the US, Trump famously tweeted: "It won't happen!"

At the weekend, Trump's National Security Adviser HR McMaster declared the North Korean advances as "intolerable, from the President's perspective".

But McMaster was more nuanced than his leader. Sure the "military option" was always on the table. But, he added: "Obviously, war is the most serious decision any leader has to make. And so, what can we do to make sure we exhaust our possibilities, and exhaust our other opportunities to accomplish this very clear objective of denuclearisation of the peninsula, short of war?".

McMaster also seemed to pull the mat from under Trump's "fire and fury" promise, declaring that the US would try all avenues before going to war, to "pressure Kim Jong-un and those around him, such that they conclude it is in their interest to denuclearise."

One of the avenues under consideration is to redeploy US nuclear weapons to South Korea, a threat that probably is directed more at Beijing than Pyongyang. China is strategically opposed to any American build-up in its regional backyard, so might be more amenable to applying greater economic pressure on Pyongyang to see the error of its nuclear ways.

Those with more skin in this game are wary of Trump's rhetoric.

South Korea's strategic reckoning assumes that any US action against the North would provoke an immediate strike by the North against the South – the consequences of which would be devastating.

Which is why South Korean President Moon Jae-in was on the blower to Trump for almost an hour on Monday.

"Above all, President Moon emphasised that South Korea can never accept a war erupting again on the Korean Peninsula," his office said in a statement about the call.

"He stressed that the North Korean nuclear issue must be resolved in a peaceful, diplomatic manner through close coordination between South Korea and the United States."

Not much fire and fury there.