South Korea will not compromise on North Korean denuclearization, the country’s president Moon Jae-in told his U.S. counterpart during a phone call on Friday.

Moon told U.S. President Donald Trump that Seoul was also prepared for the possibility that the upcoming negotiations with North Korea over its nuclear weapons might fail, according to the Nikkei Asian Review.

A White House release on the same conversation said the two leaders would learn “from mistakes of the past” in their planned summits with North Korea.

“The two leaders agreed that concrete actions, not words, will be the key to achieving permanent denuclearization of