The Ulchi Freedom Guardian (UFG) joint military drill is a computer-assisted simulation exercise which will run until Aug 29, reports Xinhua.
It will mobilize around 50,000 South Korean troops and 30,000 US forces, including 3,000 US soldiers operating overseas and coming here for the drill.
This year's UFG drill will adopt the "tailored deterrence strategy" for the first time which was introduced in October 2013 to respond to the North Korea's possible threats of nuclear weapons and weapons of mass destruction.
The South Korean military said the drill is annually held but North Korea denounced it as a rehearsal for invasion.
North Korea's General Staff of the Korean People's Army said Sunday that it will launch preemptive attacks at any time against the joint military exercise, calling it a "rehearsal for a nuclear war".
North Korea fired five short-range projectiles into its eastern waters Thursday.