Any new military conflict with North Korea would likely escalate to the use of nuclear weapons, bringing catastrophic casualties not seen since World War II, defence officials and analysts say.

While the United States has maintained an uneasy calm with North Korea for more than six decades and spikes in tension are not new, recent supercharged rhetoric between US President Donald Trump and North Korean leader Kim Jong-un have heightened the risk of miscalculation that could make an unprecedented conflict a reality, they say.

On Thursday, North Korea upped the ante by saying it would complete plans by mid-August to fire four intermediate-range missiles over Japan to land near the US Pacific island territory of Guam, after Trump said any threats by Pyongyang would be "met with fire and fury like the world has never seen".

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The exchange followed a United Nations resolution tightening sanctions on North Korea after it tested two intercontinental ballistic missiles designed to carry nuclear warheads to the US.

Trump said on Thursday his fire and fury comment was not tough enough.

Despite the war of words, the US military says, for now, there has been no change in its readiness posture in South Korea or elsewhere in Asia.

Analysts say they have seen no evidence of any increased alert in North Korea, but they warned the bluster could raise the risk of miscalculation that could result in a conflict far beyond the scale of the 1950-53 Korean War.

That conflict claimed the lives of millions of Koreans and more than 50,000 Americans and ended in an armed truce, not a peace treaty.

"The major thing people are talking about is miscalculation," Philip Yun, a Korea expert, told Reuters news agency. "We could easily stumble into something with the rhetoric being so heated."

Yun, who was an Asia adviser under former President Bill Clinton, said the risks were exacerbated by the "credibility problem" Trump has acquired because of his frequent off-the-cuff remarks that often appear to counter the more measured remarks of his officials.

"In nuclear deterrence, credibility is everything and there's a situation that if no one takes you seriously, you have to do something to make sure you are taken seriously, and that's where the miscalculation can happen," Yun said.

'Catastrophic' casualties

With hundreds of thousands of troops and huge arsenals arrayed on both sides of a tense demilitarised zone, the Korean peninsula has long been a tinder box.

North Korea's acquisition of nuclear weapons and its development of an array of missiles to deliver them have raised the stakes further.

Even a conventional clash could cause catastrophic casualties, given the thousands of North Korean artillery pieces ranged along the border.

At least 1,000 of those weapons are capable of reaching the densely populated South Korean capital Seoul and its metropolitan area, home to some 25 million people.

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"It would be very difficult to eliminate that threat before the artillery fire could create a lot of damage on the southern side," said David Shear, who served as the senior US defence official for east Asia under former President Barack Obama.

"I take projections of casualties of thousands to tens of thousands quite seriously, and that's just in South Korea," he said. "It's possible North Korea could attack Japan as well."

The real danger of any pre-emptive US strikes against North Korea's weapons' sites would be that Pyongyang might resort to using its chemical and biological weapons - and ultimately its nuclear arsenal.

There is also the potential for casualties running into the millions.

"If they did launch they could potentially wipe out cities in South Korea and Japan, and in the longer term maybe reach the US West Coast and even further inland," said Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists.

Even if everything went right for the Pentagon, a US strike campaign against North Korea would take up to a week to be effective, said Kristensen.

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Yun said the catastrophe would not just be human, but would take an economic toll on the countries as well.

"If we had a war, think about what it means," he said. "You are talking South Korea, the 11th largest economy in the world, Japan, the third largest economy, and you are talking about ground troops on the Korean peninsula.

"Donald Trump's agenda would be consumed by this. Nothing else would get done. It's against his interest and it's not really an option."