North Korea's determined quest for a nuclear-tipped missile capable of hitting the US mainland is the first major US foreign policy challenge of the Donald Trump era.

And, less than three weeks before he takes office, Trump has already plunged into these most dangerous of waters with a warning to unpredictable dictator Kim Jong-un.

Kim marked the New Year by announcing that North Korea plans to test launch an intercontinental ballistic missile (ICBM) of the kind he would need to threaten US soil.

The US president-elect responded with one of his trademark Twitter taunts, vowing to halt Pyongyang in its tracks.

"North Korea just stated that it is in the final stages of developing a nuclear weapon capable of reaching parts of the US," he declared. "It won't happen!"

Trump didn't provide any context for his promise but, if Kim continues to plough ahead despite the sanctions already imposed on his regime, the endgame is ominous.

"Has our next commander-in-chief issued, 18 days before his inauguration, a pledge that the US will wage pre-emptive war against the DPRK?" asked Strobe Talbott.

Talbott, president of the Brookings Institution and a former deputy secretary of state, spoke for many worried experts who fear Trump has limited diplomatic options.

US President Barack Obama's outgoing administration has pursued a policy of UN-backed sanctions targeting Kim's regime, and a call for six-party negotiations.

- Inevitable war? -

These talks would see North Korea come to the table with China, the US, South Korea, Japan and Russia to negotiate an end to the stand-off and a nuclear-free peninsula.

But, aside from China, outside states have little leverage over the pariah regime and Beijing opposes any stronger measures that might threaten to destabilize its neighbor.

Trump suggests an ICBM in the hands of an aggressive North Korean despot -- still technically at war with the United States since the 1950-53 war -- would be intolerable.

So, if sanctions don't work, is war inevitable?

Perhaps not yet, but the US military -- which has just under 30,000 troops in South Korea -- has stepped up planning for any eventual operation.

"It is the threat that keeps me awake at night," a senior defense official said. "Primarily because we don't know what the dear leader in North Korea is really after."

The senior official, speaking last month on condition of anonymity said US commanders have been reviewing options for 70 years but that the ICBM threat has focused minds.

Robert Einhorn, who until 2013 was State Department special adviser for non-proliferation and arms control, told AFP that Kim's threat to test an ICBM was not new.

"Whether they can deliver it is another story," he said.

"Many experts believe that the North may be two or three years away from having the ability confidently to deliver a nuclear payload on the continental US."

China does not want North Korea to join the small club of nations that can launch nuclear weapons half-way round the planet, but it doesn't want the regime to collapse either.

If sanctions cause Kim's authoritarian state to fall apart, China could face millions of refugees and see its neighbor Korea reunified as a US military ally.

Some once argued that Kim and his equally isolated predecessors were merely brandishing the nuclear threat to force the United States into direct negotiations.

But experts now see the pursuit of nuclear missiles as a strategic choice to deter South Korean or US aggression.

"I don't think these programs are any longer a bargaining chip. If they ever were," Einhorn said, adding that Trump will have to decide whether to seek direct contact.

- Trump unpredictable -

As with the 2015 Iran deal, any talks would have a multilateral veneer but the key elements would have to be worked out between US and North Korean diplomats.

For Einhorn, Trump will have to decide "whether the strategy will be pressure alone or whether pressure will have to go hand in hand with negotiations."

Are plans being laid for any of this? It's hard to tell from a single president-elect tweet, but absent any new plan, fears of war will continue to build.

"Kim Jong-un is the more dangerous," Einhorn said. "But the more unpredictable, at this stage, may be Donald Trump."