Kim Jong Un
Kim Jong Un Reuters

US President Donald Trump's national security adviser, Robert O’Brien, on Sunday warned North Korean leader Kim Jong Un against conducting a ballistic missile test as a “Christmas gift" for the US.

Speaking on ABC News, O’Brien said that Trump’s engagement in "high-level" diplomacy "over the years" with Kim has improved relations and suggested that Kim may have reconsidered his threat of a "Christmas gift" to the US.

"I think the president has engaged in personal diplomacy at a very high level with him over the years. And they have a good relationship personally. So perhaps he's reconsidered that. But we will have to wait and see," O’Brien said in the interview.

As for whether or not there will be consequences if North Korea does test long range missiles again, O'Brien said that the US will reserve judgment and take action if necessary.

"If Kim Jong Un takes that approach, we will be extraordinarily disappointed, and we will demonstrate that disappointment," he stressed.

North Korea has threatened to take unspecified action if US sanctions are not eased by the end of the year, and speculation has centered on the possibility of a new missile test, possibly of an intercontinental ballistic missile capable of delivering a nuclear warhead.

On Saturday, Kim convened a key meeting of top ruling party officials ahead of the deadline to the US.

O’Brien said on Sunday the White House believes the diplomatic process with Pyongyang can still have success.

"Kim Jong Un promised to denuclearize the Korean peninsula and we want to hold him to that commitment and we hope he follows through with the commitment that he made in Singapore," he said.

Kim and Trump engaged in months of mutual insults and threats of devastation in 2017, sending tensions soaring before a diplomatic rapprochement the following year.

The pair have met three times since June 2018, most recently in Vietnam in February, but with little progress towards denuclearization. Pyongyang has set Washington a December 31 deadline to make new concessions to kickstart stalled talks.

Since that June meeting, North Korea has conducted several tests of ballistic missiles.

Trump warned recently that Kim had "everything" to lose through hostility towards the United States.

This week, he was asked about the North Korean deadline and replied, “Maybe it’s a present where he sends me a beautiful vase as opposed to a missile test. I may get a nice present from him. You don’t know. You never know.”