The United States would be very disappointed if North Korea tested a long-range or nuclear missile and would take appropriate action as a leading military and economic power, White House national security adviser Robert O'Brien said on Sunday.

Washington has many “tools in its tool kit” to respond to any such test, O'Brien said in an interview with ABC's “This Week.”

“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.”

North Korea has asked Washington to offer a new initiative to iron out differences over Pyongyang's nuclear weapons program. It warned Washington this month that failure to meet its expectations could result in an unwanted “Christmas gift.”

U.S. military commanders have said that the North Korean move could involve the testing of a long-range missile – something North Korea has suspended, along with nuclear bomb tests, since 2017.

O'Brien said the United States and North Korea had open channels of communication but did not elaborate. He said Washington hoped North Korean leader Kim Jong Un would live up to his commitments to denuclearize the Korean Peninsula.

The United States was still the leading military power in the world and had tremendous economic power, said O'Brien. “There's a lot of pressure that we can bring to bear,” he said.