South Korea's military is 20 percent smaller than it was six years ago as the world's lowest birth rate shrinks the pool of men available for enlistment, according to a new report from the country's defense ministry.

North Korea's defense chief on Monday condemned an upcoming large-scale joint military exercise between the United States and South Korea and warned of "negative consequences."

The shortfall, driven by the Asian country’s record-low birthrate of 0.75, left the military 50,000 soldiers under the level deemed necessary for defence readiness in 2025, the defence ministry said in a report on Sunday. More worryingly, it noted, there was a shortage of 21,000 non-commissioned officers.