The 10-day military exercise is one of a series being staged by the South either alone or jointly with the United States in the aftermath of one of its warships in March.
Pyongyang warned on Sunday, Aug 15 that its army and people would "deal a merciless counterblow" to the United States and South Korea over the war games.
The massive military drill involving some 30,000 US troops and 56,000 South Korean troops began on Monday, Aug 9, a spokesman for South Korea's defence ministry said.
The sinking of the corvette the Cheonan in March with loss of 46 lives sharply raised tensions on the Korean peninsula, with a multinational inquiry determining that it was caused by a North Korean torpedo.
US military website, General Walter Sharp, who heads some 28,500 US troops based in the South, described the exercise as "one of the largest joint staff directed theatre exercises in the world."