"He definitely likes sending rockets up, doesn't he? That's why I call him 'Rocket Man'," Mr Trump said on Tuesday (Dec 3).

"We have the most powerful military we've ever had, and we're by far the most powerful country in the world," he added. "And, hopefully, we don't have to use it, but if we do, we'll use it. If we have to, we'll do it."

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On Thursday, satellite images emerged indicating North Korea may be preparing to conduct rocket engine tests at the Sohae Launch Facility, which Mr Kim once said had been dismantled in a concession to President Trump.

"Looks like North Korea is going to resume rocket engine testing at Sohae. This isn't the 'Christmas gift' the North Koreans have promised, but it is a lump of coal in Trump's stocking," tweeted Dr Jeffrey Lewis, director of the East Asia Nonproliferation Programme at the Middlebury Institute of International Studies.

WASHINGTON - Increasingly bellicose rhetoric from North Korea has raised the spectre of a long-range ballistic missile or atmospheric nuclear test by Pyongyang, perhaps as early as the New Year as it tries to force concessions from the United States in return for steps towards denuclearisation.

Earlier this week in London, President Donald Trump was distinctly less effusive about North Korean leader Kim Jong Un than he has been since they met and, in Mr Trump's words, "fell in love" in Singapore in June 2018 - and twice subsequently.

"He definitely likes sending rockets up, doesn't he? That's why I call him 'Rocket Man'," Mr Trump said on Tuesday (Dec 3).

"We have the most powerful military we've ever had, and we're by far the most powerful country in the world," he added. "And, hopefully, we don't have to use it, but if we do, we'll use it. If we have to, we'll do it."

North Korea, which has been regularly testing shorter-range missiles, had earlier warned of  an undefined "Christmas present" for the United States.

After Mr Trump's remarks, a North Korean official fired back swiftly, saying should President Trump call Mr Kim "Rocket Man" again, that  would represent a "very dangerous challenge".

On Thursday, satellite images emerged indicating North Korea may be preparing to conduct rocket engine tests at the Sohae Launch Facility, which Mr Kim once said had been dismantled in a concession to President Trump.

"Looks like North Korea is going to resume rocket engine testing at Sohae. This isn't the 'Christmas gift' the North Koreans have promised, but it is a lump of coal in Trump's stocking," tweeted Dr Jeffrey Lewis, director of the East Asia Nonproliferation Programme at the Middlebury Institute of International Studies.

In April, North Korea gave the United States  a year-end deadline to pitch for a deal on denuclearisation - a deadline the US has dismissed.

Since then, signals from North Korea - including successive visits, the latest just days ago,  to the sacred Mount Paektu by Mr Kim on a white horse - indicate that the enigmatic leader may be preparing for a major event, potentially coinciding with his traditional New Year speech on Jan 1.

President Trump faces three options before the year is out, Dr Duyeon Kim, senior adviser for North-east Asia and nuclear policy at the International Crisis Group, wrote in a Nov 28 article for CNN.

"He could try to buy time by sending another 'love letter' to Kim Jong Un; hold another summit with the North Korean leader; or insist on continuing with negotiations at the working level," she wrote.

"Each option comes with risks. The third option is the best, but Trump should intervene with another letter if a breakthrough or progress can't be made next month, in order to keep diplomacy open."

"Another letter from Trump to Kim would give both sides some time to iron out how to advance  their June 2018 Singapore summit statement on achieving a new relationship, peace, and denuclearisation and prevent a possible North Korean provocation," Dr Kim wrote.

The North Korea issue is thus once again heading towards the edge of a cliff. But that depends on the perspective.

"It's headed in a wonderful direction - for North Korea," says Dr Sung-Yoon Lee, professor of Korean Studies at the Fletcher School of Law and Diplomacy.

North Korea's messaging about a new path in 2020 if the United States does not come up with an offer conducive to starting denuclearisation, may indicate that a missile test over Japan, or a medium or intermediate-range ballistic missile test, may be on the way, he said.

But an atmospheric nuclear explosion may be more likely, since North Korea's foreign minister had made that explicit threat in September of 2017.

"I take that threat seriously, because there's a need to demonstrate to the world, and particularly the United States, that it has the capability to do that, that it is a continual, credible nuclear threat to the US mainland," Dr Lee said.

Dr Go Myong-Hyun, research fellow at the Asan Institute for Policy Studies in Seoul, told The Straits Times North Korea was likely to conduct a test, possibly of a long-range missile, but would be careful not to provoke President Trump to the extent that he would have no other option but to retaliate militarily.

"He (Mr Kim) will always leave two options for President Trump to choose from - one military, one dialogue," Dr Go said. "North Korea is very crafty, very calculating with risk. Despite the fact that North Korea is making all these threats, at the end of the day they will try to moderate."

"The chances of a major catastrophic provocation breaking out in the Korean peninsula is actually lower than perceived," he said.