The 11-day drills, dubbed Ulchi Freedom Guardian, are annual computer-simulated war games that involve about 56,000 South Korean soldiers and 30,000 U.S. troops in South Korea and abroad, South Korea's Defense Ministry and the U.S. command in Seoul said Monday.
They follow massive joint naval drills last month off South Korea's east coast that Washington and Seoul said were a show of unity following the deadly sinking of a South Korean warship in March. The allies blame North Korea for the torpedo attack that killed 46 sailors. Pyongyang denies involvement.
North Korea vowed harsh retaliation for those drills as well. Pyongyang has for years threatened the South with destruction, though it has never followed through with an all-out military assault since the Korean War ended in 1953.
The Korean peninsula technically remains in a state of war because that conflict ended with an cease-fire, not a peace treaty. The U.S. stations about 28,500 troops in South Korea and tens of thousands more in the region.
Seoul and Washington say the routine military drills are purely defensive, while North Korea calls them preparation for an attack.
"The projected war maneuvers under the spurious signboard of 'defense' are belligerent and undisguised (North Korea)-targeted war exercises," Pyongyang's military said in a statement carried Sunday by the official Korean Central News Agency.
North Korea's military will deal a "merciless counterblow" to the U.S. and South Korea, "the severest punishment no one has ever met in the world," it said.
The statement did not elaborate on how the North would retaliate. South Korea's Defense Ministry said Monday it was keeping a close watch on North Korea and had spotted no suspicious activity.
The Ulchi Freedom exercises are designed to improve the allies' joint capability to defend the South and respond to any potential provocations, the U.S. military said in a statement last month.
Earlier this month, South Korea also carried out its own naval drills near the tense western Korean sea border where the Cheonan warship went down, drawing a barrage of artillery rounds from North Korea.