The escalating crisis over North Korea is worrying experts who say that a previously unthinkable nuclear conflict is fast becoming a real possibility.

"Right now, I would say we are heading to war," said Joe Cirincione, president of the Ploughshares Fund, an organization that supports nuclear weapon reduction and eventual elimination.

Anxiety is especially acute among liberals who fear that President Trump's confrontational rhetoric could upset the balance that has maintained a tense peace on the Korean peninsula for decades.

They were unnerved by Trump's speech to the United Nations this week, in which he derided North Korean leader Kim Jong Un as "Rocket Man" and said Kim was "on a suicide mission for himself and for his regime."

Trump added that the United States would "totally destroy" North Korea in the event of an attack from Pyongyang on the U.S. or its allies.

Kim hit back with a rare personal statement, in which he called Trump a "mentally deranged dotard" and threatened "results beyond his expectation" by way of retribution.

North Korea's foreign minister suggested soon afterward that the regime was contemplating detonating a hydrogen bomb in the Pacific Ocean.

Such a move would dramatically ratchet up the confrontation. No nation has detonated a nuclear device in the earth's atmosphere since China did so in 1980.

Experts critical of Trump fear disaster could lie ahead.

"By any measure, President Trump's quote-unquote 'strategy' is not working," said Michael Fuchs, who was a deputy assistant secretary of State during the Obama administration.

"This is squarely in the hands of the president himself," said Fuchs, who is now a senior fellow at the liberal Center for American Progress. "The rest of his administration, so far at least, seems willing to carry out a strategy focused on deterrence, reassurance, pressure and diplomacy. But the president from time to time lobs a grenade on top of these efforts and spins the escalation up again."

Supporters of the president, and those who take a more hawkish line on North Korea generally, argue that tough words backed up by action are just what is required.

On Thursday, Rep. Duncan Hunter (R-Calif.) suggested in an interview with a San Diego television station that the North Koreans might soon be able to strike the West Coast of the United States with a nuclear weapon.

"The question is, do you wait for one of those? Or two? Do you pre-emptively strike them? And that's what the president has to wrestle with," Hunter said. "I would pre-emptively strike them. You could call it declaring war, call it whatever you want."

Similar notes have been sounded by John Bolton, who served as U.S. ambassador to the United Nations during the presidency of George W. Bush.

"We're getting very close to that point," where a pre-emptive strike ought to be considered, Bolton said during an interview with Leland Vittert, of Fox News Channel, on Friday.

Experts sympathetic to the line pursued by Trump argue that the threat from North Korea has existed for decades - and has been exacerbated by a lack of meaningful action against it.

Dean Cheng, a senior research fellow at the conservative Heritage Foundation, lamented that Pyongyang had "not suffered a bit of meaningful punishment" for a long history of misdeeds including assassination attempts against the leadership of South Korea, the sinking of South Korean ships and the deaths of American citizens.

Cheng added, "We, the rest of the world, have trained the North Korean leadership - not just in the case of Kim Jong Un, but for three generations - that they basically can get away with anything."

There is widespread agreement that the Trump administration's best option in the immediate future is to escalate pressure on Kim's regime, short of military action.

The president expanded sanctions this week, seeking to force international banks to cease all significant interaction with North Korea.

Treasury Secretary Steven Mnuchin told reporters in New York on Thursday that the administration would also freeze the U.S. assets of "anyone conducting significant trade in goods, services or technology with North Korea."

More liberal voices suggest the Trump administration needs to be prepared to negotiate in a meaningful way with North Korea.

"We should do what the president says he is going to do - maximum pressure and maximum engagement," said Cirincione. "We have the pressure. It's time for the engagement. You have got to walk back from the brink here. You've got to start talking to North Korea to develop a compromise."

Such a move, Cirincione said, would lower the risks of a "stumble into war."

Fuchs argued that, "We know how to keep the peace, but we are now raising the chances of another Korean War."

But, for now, Trump appears convinced that the best path is to project strength in the face of North Korean threats.

On Friday morning, he tweeted, "Kim Jong Un of North Korea, who is obviously a madman who doesn't mind starving or killing his people, will be tested like never before!"

The world is watching to see what that means.

The Memo is a column reported by Niall Stanage, primarily focused on Donald Trump's presidency.