Ban, who served as Seoul's top diplomat before assuming the top UN post, expressed concern that the liberal Moon may take a "soft" approach on the nuclear issue in consideration of "delicate" ties between the two Koreas still in a state of an armistice.

"The South-North summit should fare well," he said, addressing a forum hosted by the Reserve Officers' Training Corps in Seoul. "Unless (we) are clear and strong in our position on denuclearization, the ball will cross into the United States. We will lose the initiative."

He pointed out that the North Korean nuclear issue is "basically a matter" of South Korea, not just between Pyongyang and Washington.

In diplomacy, Ban added, those with unswerving principles win eventually.

On China, he voiced worries about a possible loophole in a web of sanctions against the North in case Beijing changes its stance a bit due to the recent summit talks between Kim and Chinese President Xi Jinping. (Yonhap)