"There's three significant risks. One is that they could actually launch one at the United States. I think that's the lowest risk. Two, the regime could fall. There's a real likelihood at some point that this regime will fall and the nuclear weapons will be out of control," Morell said.

"They'll be out there, and they'll be available to people to grab them. And then, three, North Korea could sell one of those nuclear weapons someday. So those are the three risks we face. It's significant," he said.

Fears of the North's nuclear and missile program have grown significantly in the US since last year as Pyongyang carried out a number of weapons tests, including two nuclear tests, at a rapid pace unheard of before.

Adding to the concerns were North Korean leader Kim Jong-un's threat last month to test-fire an intercontinental ballistic missile capable of reaching the US, and last week's test-firing of an intermediate-range ballistic missile.

Reflecting such concern, US President Donald Trump has called the North a "big, big problem," and a "really really important"

subject, and pledged to deal with it "very strongly." He has also said that defending against the North's nuclear and missile threats is a "very very high priority."

"The situation has become much more dangerous over the last five years or so for two reasons. One is the -- the stockpile -- the number of nuclear weapons that North Korea has, has become much larger. They keep on adding to it," Morell said.

"And, two, by testing missiles over and over and over again, those missiles are becoming more capable. And they've had more time to be able to mate a nuclear weapon to one of those missiles. So the situation is more dangerous," he said.

Former National Security Advisor Tom Donilon also said the North could be the first crisis of 2017.

"We haven't had a crisis yet, right? This, I think, could be the -- could be the first crisis in 2017," he said on the same program. (Yonhap)