North Korean leader Kim Jong Un stressed his order of a quick expansion of his forces' nuclear capabilities, citing the annual U.S.-South Korea military exercises, according to the country's state media.

Pyongyang regularly criticizes such drills as rehearsals for invasion and sometimes responds with weapons tests, but Seoul and Washington say they are purely defensive.

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.