Former Secretary of Defense Jim Mattis said political disagreements and growing "contempt" among Americans is one of the biggest national security threats to the country, during a Sunday interview on "Face the Nation."

"Internally, my bigger concern is twofold -- it's our growing debt that we're going to transfer to the younger generation... and more than that it’s the lack of friendliness," he said. "It's the increasing contempt I see between Americans who have different opinions."

"If we want this country to survive, we're going to have to work together. There's no way around that. That’s the way a democracy is set up," Mattis added.

JIM MATTIS RESPONDS TO RAND PAUL CRITICISM OVER HIS ROLE IN AMERICA'S ENDLESS WARS

He also cited Russia and China as major threats to the U.S. and said the two nations are attempting to exert their influence over other countries, by ruling with an iron fist.

"The biggest national security threat can be broken into two segments. One is external, and clearly those nations, Russia and China, that are trying to impose their authoritarian models and decisions over other countries," he said earlier in the interview.

"Whether it be in the South China Sea or in the Ukraine -- in parts of Georgia that Russia's occupied. They've mucked around in our elections... That’s why we rewrote the national defense strategy to acknowledge the reality of those nations."

CLICK HERE FOR THE FOX NEWS APP

Mattis released a book titled "Call Sign Chaos" on Tuesday, where he does not reference his time in the Trump administration but instead chronicles his career in the Marine Corps ending with his eventual rise to the head of U.S. Central Command.