Tension has been high since North Korea shelled the South's Yeonpyeong island last month, killing four South Koreans. The North has branded the exercises "warmongering" but its criticism has been more muted than normal.
The South Korean army has made no secret of the fact the drill aims to display its full firepower to its neighbour. Thursday's drill is being held at Pocheon, about 50km from central Seoul. Exercises have been held at Pocheon before, but this is on an unprecedented scale, an army spokesman said.
Although the South has conducted no fewer than 47 military drills this year, this is the largest winter live-fire exercise ever conducted on land. It began about 45 minutes later than the scheduled start time of 1400 local time (0500 GMT) and was to last about an hour, with more than 100 types of weapons deployed, including tanks, anti-tank missiles, helicopters and fighter jets.