North Korean leader Kim Jong Un said a U.S.-South Korea joint military exercise could "ignite a war" and called for the rapid expansion of Pyongyang's nuclear capabilities.

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

South Korea’s birth rate has dropped throughout much of the past decade, spelling trouble for the military as regional threats and global conflicts simmer.