Commenting on Donald Trump’s plan to arm Japan and South Korea with nuclear weapons, Secretary of State John Kerry said, “I can’t think of anything that would be more volatile.”

Secretary of State John Kerry ripped Donald Trump on foreign policy during an interview with journalist Charlie Rose on PBS, saying that Trump’s plan to allow Japan and North Korea to have nuclear weapons is “beyond provocative.”

Kerry repeated previous assertions that world leaders tell him they’re “nervous” about the future of U.S. foreign policy because of Trump, and he singled out one of Trump’s policy proposals.

“When Donald Trump talked about Korea and Japan going out and getting their own nuclear weapons,” he told Rose, “I can’t think of anything that would be more volatile, more contrary to peace and stability in the region, more contrary to the fundamental commitment of every president since World War II to try to minimize the risk of nuclear weapons and minimize the number of people who have them.”

Kerry added: “And here’s a guy running for president who just says, ‘oh, let them go get them themselves’ and it’s beyond provocative.” Watch:

Trump’s (misguided) complaint is that the United States pays too much for Japanese and South Korean security and gets nothing in return. So in a recent interview, he said he wouldn’t mind if those two countries developed nuclear weapons to defend themselves.

Nuclear and non-proliferation experts lampooned Trump’s plan, calling it “irresponsible,” a “total catastrophe,” and saying it “would sound half-clever if he was sitting on a bar stool.”

Joe Cirincione, president of the Ploughshares Fund, said Trump’s idea “would reverse course” in ridding the world of nuclear weapons and “roll back the progress we have made over the past decades, ending any hope of reducing nuclear dangers.” He added, “We are already on the brink of a nuclear arms race. He would hit the accelerator.”

What’s more, the Japanese and South Koreans themselves were “dumbfounded” by Trump’s plan and have no interest in building nuclear weapons.

When asked whether Japan and South Korea obtaining nuclear weapons could spark a war with North Korea, Trump said, “it would be a terrible thing but if they do, they do.”

This isn’t the first time Kerry has gone after Trump on foreign policy. On Tuesday, he said, “Every meeting I have, everywhere, people are asking what is happening with the United States, ‘What are you doing to yourselves?'” calling the GOP side of the presidential race “embarrassing.”

Kerry also said Trump’s call to ban Muslims from entering the United States would “endanger national security.”