SEOUL—South Korean President Moon Jae-in on Thursday warned North Korea towards conducting any armed attack on Seoul subsequent many weeks of rising tensions that have dragged inter-Korean relations to their least expensive position in decades.

Mr. Moon’s stern remarks had been unusually strong for a president who has devoted the past 3 decades to improving relations with Pyongyang. Throughout that period of time, he engaged North Korean leader Kim Jong Un in 3 summits and in 2018 facilitated the initial summit involving a sitting U.S. president…