Mullen: North Korea More Likely Than Any Other Foreign Policy Challenge to Have 'Explosive Outcome'

"I think actually this has to go through Beijing. And Beijing has pushed back on that time after time... we're going to get to the point where he's going to be able to put a nuclear weapon on top of an intercontinental ballistic missile that could hit the United States, and that's unacceptable."

Mullen said retired Lt. Gen. Michael Flynn, former director of the Defense Intelligence Agency and a strong Trump supporter said to be the president-elect's pick for national security advisor, was a "terrific intelligence officer," but "now he's at the hub of the national security apparatus and he's got to be an honest broker."

"He's got to present all opinions. He has to actually -- he and his deputy have to make the trains run on time in the White House," Mullen said. "The number of issues are extraordinary and he has got to be able to do that to give the president and the cabinet, but really the president, all the options in a way where the president can make the best decision for the security of the country."