Kim said the "security environment" around North Korea is "getting more serious day by day" and "requires us to make a radical and swift change in the existing military theory and practice and rapid expansion of nuclearization," the official Rodong Sinmun newspaper reported, paraphrasing his remarks.

North Korea's Supreme Leader Kim Jong Un remained unhappy as the U.S. and South Korea conducted their annual summer joint military drills.

North Korea must grow its nuclear arsenal, Kim Jong-un has said, as he hit out at joint military drills between the US and South Korea.