WASHINGTON (BLOOMBERG, REUTERS) - Senator John McCain, the Republican chairman of the Senate Armed Services Committee, said US President Donald Trump needs to be more cautious in his statements because he may not be able to make good on his North Korea threat.

"The great leaders I've seen don't threaten unless they're ready to act, and I'm not sure President Trump is ready to act," McCain said in an interview on Phoenix radio station KTAR.

McCain said on Tuesday that President Trump should not threaten North Korea with "fire and fury" unless he is prepared to carry out those threats.

"I don't know what he's saying and I've long ago given up trying to interpret what he says," the Senator told KTAR.

McCain said he didn't know how seriously to take the president's warning. "It's the classic Trump in that he overstates things."

He noted, however, that Trump's remarks could be pivotal in escalating a confrontation with North Korea, which could ultimately endanger South Korea in what he said could be a catastrophic scenario.

"They have 1,000 rockets aimed at Seoul that could set that city on fire," he said.

Were he advising the President on this issue, McCain said, he would tell Trump to speak with the Chinese and accelerate the United States' anti missile defence capabilities.

"The key to this is China," McCain said.

When asked to rate the North Korean threat on a scale of one to 10, McCain said, "I'd say 6, 6 or 7."

President Trump ratcheted up his rhetoric against North Korea to an unprecedented level on Tuesday, warning Kim Jong Un's regime will face a devastating military strike if it continues threatening the US.

"North Korea best not make any more threats to the United States," Trump told reporters at his golf course in Bedminster, New Jersey.

"They will be met with fire, fury and, frankly, power the likes of which this world has never seen before." The US military's newspaper Stars and Stripes said his comments appeared scripted, with Trump glancing at a paper in front of him.

Trump's comment came as North Korea, reacting to new UN sanctions against its rapidly developing nuclear weapons programme, warned the US it would "pay dearly" for its crimes and said it was examining its plans to strike the American military base of Guam with a missile.

It also followed a report in the Washington Post, citing a Defence Intelligence Agency analysis, that Pyongyang successfully developed a nuclear warhead that could fit onto its missiles.

"He has been very threatening, beyond a normal statement," Trump said of Kim. North Korea has routinely used bombastic rhetoric to threaten the US, including military bases in Japan, South Korea, Guam and Hawaii.

It was not clear whether the president consulted with key allies, the Pentagon or State Department before making his comment.

It was also unclear whether Trump's "fire and fury" threat meant an overwhelming conventional strike or the use of nuclear weapons.

The Defence Department didn't offer an explanation.

"We remain prepared to defend ourselves and our allies and to use the full range of capabilities at our disposal against the growing threat from North Korea," Johnny Michael, a Pentagon spokesman, said in an e-mail.

North Korea's reported progress on miniaturising nuclear warheads, coupled with two test flights of intercontinental ballistic missiles in July, are adding to the pressure on Trump, who vowed during last year's presidential campaign that North Korea wouldn't develop a nuclear warhead capable of reaching the US: "It won't happen," he wrote on Twitter.

Now, Trump may have decided it was time to strongly signal to Kim that U.S. patience is running out.

In doing so, Trump "runs the risk of entrapping" himself in his own rhetoric, said Scott Snyder, director of the programme on U.S.-Korea policy at the Council on Foreign Relations. "If he pursues that, it's hard for me to see how it can accomplish anything positive," he said.

Former US diplomat Douglas Paal, now with the Carnegie Endowment for International Peace think tank in Washington, said Trump should not get into a war of words with Pyongyang.

"It strikes me as an amateurish reflection of a belief that we should give as we get rhetorically. That might be satisfying at one level, but it takes us down into the mud that we should let Pyongyang enjoy alone," said Paal, who served as a White House official under previous Republican administrations.

Representative Eliot Engel, the top Democrat on the House Foreign Affairs committee, said Trump's latest comments "undermined American credibility by drawing an absurd red line".

Kim's efforts to develop a missile capable of delivering a nuclear weapon to the continental US still face significant technological hurdles.

Air Force General Paul Selva, vice-chairman of the Joint Chiefs of Staff, highlighted those issues in an Aug 3 presentation. They include developing a missile that can survive reentry into the atmosphere and a guidance-and-control system capable of directing a rocket all the way to the US without breaking up.

The missile tests show Kim's regime making important progress. Trump's comments may be "emotional rhetoric expressing unhappiness with what had gone on over past 10 years," said Paul Bracken, a professor of political science and management at Yale University and author of The Second Nuclear Age: Strategy, Danger, and the New Power Politics. Just days ago, Trump was praising the UN Security Council's unanimous decision to tighten sanctions on North Korea, targeting about US$1 billion of the nation's approximately US$3 billion in exports. Some analysts say those measures are too little, too late to slow Kim's momentum.

"Today North Korea became only the third US adversary, after Russia and China, to attain the ability to threaten nuclear war against the United States," said Matthew Kroenig, a non-resident senior fellow with the Brent Scowcroft Center on International Security at the Atlantic Council, referring to the Defence Intelligence Agency report.

"This news will likely provide motivation for the Trump administration to take a hard look at higher-risk options, including military force." Kim shows no sign of being cowed by condemnation or threats. The regime has long viewed its nuclear weapons program as a means to winning respect and ending any potential threat to oust the Kim dynasty, which has ruled the nation for three generations.

"We will, under no circumstances, put the nukes and ballistic rockets on negotiating table," the state-run Korean Central News Agency reported on Monday.

"Neither shall we flinch even an inch from the road to bolstering up the nuclear forces chosen by ourselves, unless the hostile policy and nuclear threat of the US against the DPRK are fundamentally eliminated."