Helicopters are mobilized for the 2017 Ulchi Freedom Guardian drill (Photo: EPA)

The military exercise will involve 50,000 South Korean soldiers and 17,500 American troops, largely involved in computer simulation. The US and South Korea said that the 10-day exercise is for defense only, but North  Korea’s state media on Sunday called the drills a "reckless" move that could trigger the "uncontrollable phase of a nuclear war."