Experts have interpreted the message delivered earlier this month by Pyongyang as a warning that a long-range missile test was imminent if the US did not give ground by the end of the year in negotiations to denuclearize the Korean peninsula.

Asked by ABC about possible consequences of such a test, White House national security advisor Robert O'Brien said he did not want to speculate.

"But we have a lot of tools in our toolkit, and additional pressure can be brought to bear on the North Koreans," he said.

O'Brien said North Korea's nuclear program was the "most difficult challenge in the world" when President Donald Trump took office in January 2017.

The aide suggested that Trump's strategy of face-to-face diplomacy may have forced North Korean leader Kim Jong Un into a rethink.

"So perhaps he's reconsidered that. But we'll have to wait and see," O'Brien said. "We're going to monitor it closely. It's a situation that concerns us, of course."

Talks on denuclearization have been largely deadlocked since a second summit between Trump and Kim in Hanoi collapsed at the start of this year.

Pyongyang has issued a series of increasingly assertive comments as its time limit approaches.

It has carried out a series of static tests at its Sohae rocket facility this month, after a number of weapons launches in recent weeks.

"We'll find out what the surprise is and we'll deal with it very successfully," Trump told reporters at his Mar-a-Lago resort in Florida on Tuesday.

"Everybody's got surprises for me, but let's see what happens. I handle them as they come along."

"Maybe it's a nice present, maybe it's a present where he sends me a beautiful vase as opposed to a missile test," the president joked.