U.S. National Security Adviser Robert O'Brien speaks during a press conference on the sidelines of the 35th Association of…
FILE - U.S. National Security Adviser Robert O'Brien speaks during a press conference on the sidelines of the 35th Association of Southeast Asian Nations (ASEAN) summit in Nonthaburi, Thailand, Nov. 4, 2019.

White House national security adviser Robert O’Brien said Sunday he did not want to speculate about North Korea and its threat of "Christmas gift," but added the U.S. would be "very disappointed" if Pyongyang tested a long-range or nuclear missile.

During an interview with ABC’s "This Week," O’Brien said the country would take appropriate action as a leading military and economic power if North Korea went ahead with such a test.

O’Brien added Washington has many "tools in its tool kit" to respond.

"We’ll reserve judgment but the United States will take action as we do in these situations," he said. "If Kim Jong Un takes that approach we’ll be extraordinarily disappointed and we’ll demonstrate that disappointment."

Watch related video by VOA's Arash Arabasadi.

North Korea had warned of a "Christmas gift" if the U.S. didn't meet an end of year deadline to soften its stance on nuclear talks that have been stalled since February

U.S.  officials  have been on alert for a potential long-range missile test since the North Korean warning.  

Though Christmas holiday has passed and North Korea did not deliver the so-called "Christmas gift" to the United States,  U.S.-North Korea tensions appear far from resolved.

North Korean leader Kim Jong Un speaks during the Third Enlarged Meeting of the Seventh Central Military Commission (CMC) of…
No North Korean ‘Christmas Gift’ Yet, But Deadline Looms
No ICBM launches or other provocations on Christmas from North Korea

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

He also suggested that Trump's strategy of "face-to-face" diplomacy may have forced North Korean leader Kim Jong Un to reconsider.

Talks about North Korea's denuclearization have been largely deadlocked since a second summit between Trump and North Korean leader Kim Jong Un in Hanoi collapsed at the start of this year.