President Trump’s national security adviser says North Korea is making a “big mistake” if it is preparing to resume tests of nuclear bombs — tests Pyongyang has refrained from conducting for the past 18 months.

National Security Adviser Robert O’Brien offered the warning Sunday after North Korea announced it had carried out an unspecified but “very important test” at a long-range rocket launch site — a move some analysts saw as a sign Pyongyang may be moving toward a more aggressive provocation in the coming days or weeks.

North Korea has made international headlines over the past week by asserting that the Trump administration is running out of time to salvage denuclearization talks that stalled months ago, saying it’s up to the U.S. to choose what “Christmas gift” it gets from Pyongyang.

Analysts say North Korean leader Kim Jong-un is trying to increase pressure on Washington in line with his end-of-year mandate that the Trump administration soften its posture toward nuclear negotiations — a mandate widely seen as a demand that the United States ease sanction against Pyongyang.

During an interview Sunday on CBS’s “Face the Nation,” Mr. O’Brien stressed that Mr. Kim had “promised to denuclearize North Korea” at the first historic summit the North Korean leader had with President Trump in Singapore in June 2018.

“We expect him to live up to the promise he made at the summit,” the national security advisor said. “We hope he’ll do so.”

During the decade leading up to the Singapore summit, Pyongyang had carried out six nuclear tests, with the first occurring in 2006, followed by tests in 2009, 2013, two in 2016, and another in 2017. Analysts say there have since been no such nuclear bomb tests by the North Koreans.

Pyongyang said Sunday that the recent unspecified test it had carried out a day earlier occurred at the Sohae Satellite Launching Ground — a long-range rocket launch site that North Korean officials reportedly rebuilt after having partially dismantled it in the wake of the Singapore summit.

According to The Associated Press, North Korean state media cited a spokesman for the isolated nation’s Academy of National Defense Science as saying the unspecified test will have “an important effect on changing the strategic position of [North Korea] once again in the near future.”

Prior to Mr. O’Brien’s comments Sunday, Mr. Trump had already reacted to reports of the North Korean test by tweeting that North Korea “must denuclearize.”

“Kim Jong Un is too smart and has far too much to lose, everything actually, if he acts in a hostile way,” Mr. Trump tweeted Sunday.

“North Korea, under the leadership of Kim Jong Un, has tremendous economic potential, but it must denuclearize as promised,” he said.

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