South Korea’s military has shrunk by 20% in the past six years, largely due to a sharp decrease in the population of men of enlistment age for mandatory service in the country with the world’s lowest birthrate, according to a report.

The size of South Korea’s military has declined steadily since the early 2000s, when it had about 690,000 soldiers. The pace accelerated during the late 2010s and there are now about 450,000 troops.

The sharp decline in the pool of men available for military service is also causing a shortfall in the number of officers and could result in operational difficulties, the defence ministry said in the report.