U.S. National Security Adviser Robert O'Brien warned on Sunday that if North Korea engages in provocation, the U.S. will be very disappointed and will demonstrate that disappointment.

During an interview with US broadcaster ABC, O'Brien said that the U.S. "always monitors the situation" in North Korea and noted that its leader Kim Jong-un "has said there would be something over Christmas."

Last month, North Korea promised to deliver a "Christmas gift" if progress is not made in negotiations between the two nations ahead of an end-of-year deadline imposed by the North. 

When asked about the consequences should North Korea resumes testing nuclear weapons or long-range missiles, O'Brien said he doesn't want to speculate about what will happen, but the U.S. has a "lot of tools in its tool kit" and additional pressure can be brought to bear on North Korea. 

The security adviser assessed that President Trump's engagement in "high-level" diplomacy over the last 2 years with Kim has improved relations and that the two leaders have a good personal relationship . 

O'Brien added that perhaps Kim has reconsidered his threat of a Christmas gift, but the U.S. will have to wait and see.