SEOUL (NYTIMES) - With North Korea's deadline for US concessions fast approaching, the North announced Sunday (Dec 8) that it had conducted a "very important test" at a missile-engine site, rapidly ramping up pressure after months of carefully calibrated provocations.

Although US President Donald Trump has generally played down the North's actions, on Sunday - just weeks before the Dec 31 deadline - he issued a strong statement of his own, posting on Twitter that "Kim Jong Un is too smart and has far too much to lose, everything actually, if he acts in a hostile way."

Analysts said North Korea had probably conducted a ground-based test of a new type of engine for long-range ballistic missiles.

But the North did not yet describe the test or release images of it, as it has in the past, so it is unclear whether it was a success.

Still, if an engine was tested, analysts said it could be a warning that Mr Kim is considering returning to long-range missile tests.

It was a series of such launches in 2017 that provoked a crisis with the United States as it became clear that the North's longest-range missiles were capable of reaching the West Coast of the United States, and perhaps beyond.

Mr Kim last year announced a halt to all tests of those intercontinental missiles and nuclear weapons, a self-imposed moratorium that Mr Trump has repeatedly cited as one of the main achievements in his on-again, off-again diplomacy with Mr Kim.

Should they resume, it could mark a complete breakdown in what Mr Trump had hoped would be his signature foreign policy achievement.

The latest provocation comes after a series of short-range missile tests and statements by senior North Korean leaders that indicate that Mr Kim is running out of patience with diplomacy that has not won him the relief he craves from crippling sanctions.

Those statements included a warning by a vice foreign minister that "it is entirely up to the US what Christmas gift it will select to get."

North Korea has not been explicit about what might happen after Dec 31, except that Mr Kim has warned of finding a "new way" if Washington persists with sanctions and tries to force an unpalatable denuclearisation deal.

But senior North Korean officials have hinted that the new approach might include resuming nuclear and intercontinental ballistic missile tests.

It has been obvious for months that the groundbreaking diplomacy between Mr Kim and Mr Trump has stalled.

Just hours before the test was announced Sunday, the North's ambassador to the United Nations said that denuclearisation was off the negotiating table - the key element of discussions between the two leaders over the course of three meetings.

On Saturday, responding to the North Korean announcement that denuclearisation was off the table, Mr Trump emphasised his new relationship with Mr Kim - seeming to suggest that would overcome the problems.

He kept repeating that he did not believe that Mr Kim wanted to "interfere" in the 2020 US elections, presumably a reference to how a return to missile launches and nuclear tests could create a confrontation just as Mr Trump was running for re-election.

By Sunday morning, Mr Trump seemed to be hardening his tone with his Twitter message, which he put in a political context.

"He does not want to void his special relationship with the President of the United States or interfere with the US Presidential Election in November."

The president also said that Mr Kim had committed himself to denuclearisation when the two men first met in Singapore, in June 2018.

But that commitment called for working "toward complete denuclearisation of the Korean Peninsula," wording that the North has used for decades to mean that it would dismantle its nuclear arsenal only if there was a complete pullback of US forces, including nuclear-capable ships, submarines and other weaponry.

The United States has never publicly discussed what it might be willing to give up and has always suggested North Korean denuclearisation must come first.

The mere announcement Sunday of the test was a sign the Singapore agreement was coming apart.

At a news conference in Singapore, Mr Trump had said Mr Kim had promised him he would dismantle the very site where the North conducted Saturday's test.

"Chairman Kim has told me that North Korea is already destroying a major missile-engine testing site," he said at the time. "That's not in your signed document; we agreed to that after the agreement was signed. That's a big thing - for the missiles that they were testing, the site is going to be destroyed very soon"

The test was carried out Saturday at the Sohae Satellite Launching Ground, also called the Tongchang-ri site, near the North Korean border with China, a spokesman for the North's Academy of National Defence Science said in a statement that was carried by the official Korean Central News Agency.

The brief statement provided no further details. The site is also used to launch small satellites aboard North Korean rockets.

But a rocket launch would have been instantly detected by US and other satellites.

The academy reported "the results of the successful test of great significance" to the Central Committee of North Korea's ruling Workers' Party, the spokesman said.

The spokesman said that the successful test "will have an important effect on changing the strategic position" of North Korea "once again in the near future."

While Mr Trump falls back on his relationship with Mr Kim - their meetings and their exchange of what he has called "beautiful letters" - even top White House aides acknowledge that progress has been stalled.

Subsequent meetings with Mr Trump, in Hanoi and in Korea, and continuing negotiations between Pyongyang and Washington have failed to resolve differences over how to carry out the broadly worded Singapore deal.

Now Mr Kim seems to be tiring of the negotiating.

In Hanoi early this year, he offered to shutter the Yongbyon nuclear site, the country's oldest and largest nuclear facility, in return for a lifting of the most onerous sanctions.

But the Trump administration has said that is insufficient because so much of his nuclear and missile capability is now outside the Yongbyon facility.

Mr Kim, who has promised his long-suffering people economic recovery, is scheduled to convene his party's Central Committee this month to "discuss and decide on crucial issues," given "the changed situation at home and abroad," according to the North Korean state news media.

In the meantime, analysts are searching for clues about the technical importance of Saturday's test.

Mr Kim Dong-yub, a North Korea expert at the Seoul-based Institute for Far Eastern Studies, said it may have involved a new type of ICBM engine that uses solid fuel.

North Korea's existing ICBMs use liquid fuel.

North Korea has been trying to convert its missiles from liquid fuel to solid, which is easier to transport.

Solid fuel missiles are also faster to launch and easier to hide, thus making it harder for the US military to find and target its missiles before they are launched.

Although US intelligence agencies estimate North Korea has 30 to 60 nuclear weapons, it remains unclear if the weapons - once loaded on a long-range missile - could withstand the heat and vibrations of reentering the atmosphere.

"North Korea is avoiding violations of its long-range missile test moratorium for now, but it is still improving the propulsion and precision of its missiles so that it can claim a credible nuclear deterrent," said Associate Professor Leif-Eric Easley, an international studies expert at Ewha Womans University in Seoul.

"The Kim regime knows that US surveillance flights and satellites are watching. So with the activity at Sohae, Pyongyang is also trying to raise international concerns that it may intensify provocations and walk away from denuclearisation talks next year."