U.S. President Donald Trump threatened North Korea "with fire and fury like the world has never seen" on Tuesday after suggestions Pyongyang has mastered one of the final hurdles to being able to strike the continental U.S. with a nuclear missile.

North Korea's army says it is studying a plan to create an "enveloping fire" in areas around Guam, a U.S. territory about 3,400 kilometres away, with medium to long-range ballistic missiles.

The army said in a statement distributed by the state-run news agency that it's examining its operational plans for attacking Guam, in order to contain U.S. military activity there. Guam is home to Andersen Air Force Base.

The competing threats escalated tensions between the foes even further. Although it wasn't clear if Trump and the Koreans were responding directly to each other, the heightened rhetoric added to the potential for a miscalculation that might bring the nuclear-armed nations into conflict.

Guam's reaction

The speaker of the Guam legislature said he hopes the island can defend itself in the event of an attack from North Korea.

"We're just praying that the United States and the ... defence system we have here is sufficient enough to protect us," said Benjamin J. Cruz in a telephone interview.

Cruz called the threat "very disconcerting."

"It forces us to pause and to say a prayer for the safety of our people."

Guam Governor Eddie Calvo dismissed the North's 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.

"Guam is American soil ... We are not just a military installation," Calvo said in an online video message.

Rena Chang, who owns a hair salon in the tourist area of Tumon, said, "That's so scary. My heart is pumping right now."

'Fire and fury'

Trump's stern words to the camera at his golf course in Bedminster, N.J., came hours after reports indicating North Korea can now wed nuclear warheads with its missiles, including those that may be able to hit the American mainland.

The isolated and impoverished dictatorship has strived for decades to have the ability to strike the U.S. and its Asian allies, and the pace of its breakthroughs is already having far-reaching consequences for stability in the Pacific and beyond.

The nuclear advances were detailed in an official Japanese assessment and a Washington Post story that cited U.S. intelligence officials and a confidential Defense Intelligence Agency report.

The U.S. now puts the North Korean arsenal at up to 60 nuclear weapons, more than double most assessments by independent experts, according to the Post's reporting.

Donald Trump warns North Korea "they will be met with fire and fury like the world has never seen." 0:32

"North Korea had best not make any more threats to the United States," said a stern-looking Trump, seated with his arms crossed and with his wife beside him. "They will be met with fire and fury like the world has never seen."

"He has been very threatening beyond a normal state. And as I said they will be met with fire, fury and frankly power the likes of which this world has never seen before."

The remarks appeared scripted, with Trump glancing at a paper in front of him. They evoked President Harry Truman's announcement of the U.S. atomic bomb dropped on Hiroshima, Japan, in 1945, in which he warned of "a rain of ruin from the air, the like of which has never been seen on this earth."

'The wrong approach'

But it wasn't clear what Trump, who is prone to hyperbole and bombast in far less grave situations, meant by the threat. White House officials did not elaborate.

A North Korean government photo shows what it says is the launch of a Hwasong-14 intercontinental ballistic missile. Reports say North Korea may have successfully produced a miniaturized nuclear warhead that can fit inside its missiles. (Korean Central News Agency/Korea News Service via AP)

"President Trump's threats and bellicose rhetoric is exactly the wrong approach," Kelsey Davenport, the director of non-proliferation policy at the Arms Control Association in Washington, told CBC News Network.

"Trump is only going to inflame tensions further with North Korea, and this will increase the chance that these tensions could boil over into conflict," she said.

The Trump administration considers North Korea to be America's greatest national security threat and tensions have steadily escalated this year.

UN imposes tougher sanctions

Pyongyang responded angrily to the UN Security Council's adoption this weekend of new, tougher sanctions spearheaded by Washington. The sanctions followed groundbreaking long-range missile tests last month that showed the North could potentially reach the continental United States with its missiles.

The newly revealed U.S. intelligence assessment indicates those missiles can carry nuclear warheads.

Denouncing the UN sanctions through state media, the North warned: "We will make the U.S. pay by a thousand-fold for all the heinous crimes it commits against the state and people of this country."

For North Korea, having a nuclear-tipped missile that could strike America would be the ultimate guarantee against invasion by its superpower adversary.

It is an ambition decades in the making. North Korea began producing fissile material for bombs in the early 1990s and conducted its first nuclear test explosion in 2006. Four subsequent nuclear tests, the latest a year ago, have accelerated progress on miniaturizing a device — something North Korea already claimed it could do. Over that span, multiple U.S. presidents have tried and failed to coax or pressure Pyongyang into abandoning its nuclear ambitions.

The secrecy of the North's nuclear program and the underground nature of its test explosions make it very difficult to properly assess its claims. But the new assessments from Japan and the U.S. suggest that doubts over the North's abilities are receding.

Miniaturization of nuclear warhead

In an annual report, Japan's Defense Ministry on Tuesday concluded that "it is possible that North Korea has achieved the miniaturization of nuclear weapons and has developed nuclear warheads." Japan, a key U.S. ally, is a potential, front-line target of North Korean aggression.

The Post story, citing unnamed U.S. intelligence officials, went further. It said the Defense Intelligence Agency analysis, completed last month, concluded North Korea has produced nuclear weapons for ballistic missile delivery, including by intercontinental missiles.

Officials at the agency wouldn't comment Tuesday. The Office of the Director of National Intelligence also wouldn't discuss the report.

Diplomacy vs. threats and sanctions

Apparently North Korea has not successfully tested if its nuclear missile can withstand the heat generated when it re-enters the atmosphere.

"So, the reliability and the accuracy of these missiles to actually reach their targets are certainly questionable," Davenport said.

"And that does give the United States time to engage in a concerted diplomatic effort to prevent North Korea from further developing these capabilities," she said.

Davenport is critical of both the Trump and the Obama administrations' approach. "The typical playbook of sanctions pressure and threats is not going to stop North Korea. The U.S. needs to negotiate with them."

"Requiring North Korea to take steps to give up its nuclear program before even engaging in talks is putting the cart before the horse."

North Korean leader Kim Jong-un reacts with scientists and technicians after the test-launch of a Hwasong-14 missile in July. (KCNA via Reuters)

It's unclear how North Korea's new capabilities will immediately affect how the U.S. approaches the country's regular missile launches and occasional nuclear tests. The U.S. military has never attempted to shoot a North Korean missile out of the sky, deeming all previous tests to pose no threat to the United States. The U.S. could weigh military action if the threat perception changes.

Calculating North Korea's nuclear arsenal

The calculation of North Korea's nuclear arsenal at 60 bombs exceeds other assessments, which range from around one dozen to about 30 weapons. The assessments are typically an estimate of the amount of plutonium and enriched uranium North Korea has in its inventory rather than how much of that material has been weaponized. It's unclear how many, if any, miniaturized warheads North Korea has built.

Last month's tests of intercontinental ballistic missiles highlighted the growing threat. While those missiles landed at sea near Japan, both were fired at highly lofted angles. Analysts said the weapons could reach Alaska, Los Angeles or Chicago if fired at a normal, flattened trajectory.

North Korea threatened to hit Guam with its Hwasong-12 missiles, which it says can carry a heavy nuclear warhead, and before the two ICBM launches had demonstrated the longest potential range of the missiles test-fired by the North.

Not all technical hurdles have been overcome with the missiles, however. North Korea is still believed to lack expertise to allow a missile to re-enter the Earth's atmosphere without burning up. Another work-in-progress: the ability to strike targets with accuracy.