Warren Buffett has repeatedly emphasized in past interviews that little can derail the U.S. economy.

But now he’s saying there’s one thing that could cause trouble: rising tensions between the U.S. and North Korea, with the potential for nuclear warfare.

“It’s the only real cloud on the horizon,” the Berkshire Hathaway CEO said of atomic bombs in a CNBC interview early Wednesday. “We’ve got a million problems always in the country of one sort or the other—they get solved. This one is the ultimate problem because if Kennedy and Khrushchev hadn’t behaved in the 1960s, there would have been millions of Americans killed. Who knows what is going to happen.”

Buffett’s response came after CNBC’s Becky Quick cited one of President Donald Trump’s early-morning tweets. That tweet appeared to suggest that diplomacy wasn’t the way to address relations between the U.S. and North Korea.

“The U.S. has been talking to North Korea, and paying them extortion money, for 25 years. Talking is not the answer!” the president tweeted after North Korea launched a missile over Japan. It’s unclear what the president meant by “extortion money.”

Experts have noted that Trump and his counterpart Kim Jong-Un in Pyongyang have been more aggressive than their predecessors in dealing with each other in recent months. While Kim has conducted about 80 missile tests as supreme leader—more than his father and grandfather combined—Trump has threatened Kim with the U.S.’s military might via tweets and ad-libbed comments.

Trump’s administration, however, has tried to strike a different tone from the president. Following Trump’s early Wednesday tweet, U.S. Defense Secretary Jim Mattis said that the U.S. is “never out of diplomatic solutions.”

Still, Buffett warned that as leaders turn over in both North Korea and the U.S., the threat of nuclear war rises.

“The more generations you go through of different leaders, the more likely eventually something happens,” he said to CNBC. “And North Korea—who knows what is in the mind of that fellow and who knows what will be in his mind five years from now.”

Read the original article on Fortune.