Ash Carter: A US pre-emptive strike on North Korea could lead to 'invasion' of South Korea Former Defense Secretary Carter also said China fears a unified Korea.

 -- Former Defense Secretary Ash Carter says a U.S. preemptive strike against North Korea’s nuclear-weapons program could prompt the country’s leader, Kim Jong Un, to try to invade South Korea.

Carter, who served as defense secretary in the Obama administration, told ABC News’ Martha Raddatz on “This Week” Sunday that if the U.S. preemptively strikes North Korea, “It's quite possible that they would as a consequence of that launch … attempt an invasion of South Korea.”

“I'm confident of the outcome of that war, which would be the defeat of North Korea,” Carter said. “But, Martha, I need to caution you, this is a war [with] ... an intensity of violence associated with it that we haven't seen since the last Korean War.”

“Even though the outcome is certain, it is a very destructive war,” the former defense secretary said, noting that the South Korean capital of Seoul, with about 25 million area residents, is on the border between the two countries. “So one needs to proceed very carefully here.”

Carter’s comments come as President Trump is set to meet this week with Chinese President Xi Jinping.

The Trump administration wants China to get North Korea to end its nuclear program.

But Carter said the Chinese have been “unwilling” to take on that role because, in the short term, they are “fearful of a collapse of North Korea or a war” that would result in a “unified Korea allied with the United States on their border.”

Carter also commented on U.N. Ambassador Nikki Haley’s taking a different, tougher tone toward Russia than President Trump himself.

"In strategic affairs, clarity and consistency are very important,” Carter said. “This is our government and our foreign policy, and we all need to wish it success. But there does need to be clarity and consistency."