The drills, due to end on Saturday, are the latest in a series of military exercises by South Korea in recent weeks, including joint activities with the United States and Japan.
The latest field training came as North Korea has been carrying out weapons tests at an unprecedented pace this year, firing a short-range ballistic missile and hundreds of artillery rounds near the heavily armed inter-Korean border on Friday.
Pyongyang has strongly reacted to the South Korean and joint military activities, calling them provocations and threatening countermeasures. Seoul says its exercises are regular and defense-oriented.
Joined by some US forces, the South Korean troops will focus on maintaining the readiness and improving the troops' ability to execute joint operations during the Hoguk drills, the South's Joint Chiefs of Staff said, Reuters reported.
Last week, tensions flared after the North fired a missile, shot more than 500 artillery shells and flew a multitude of warplanes near the skirmish-prone sea border.
Seoul condemned Pyongyang and imposed its first unilateral sanctions in nearly five years, describing the moves as a violation of a 2018 bilateral military pact.
But the North accused the South's military of escalating tension with its own artillery firing.
MP/PR