A spokesman for the South Korean Joint Chiefs of Staff said on Monday that the Key Resolve military exercise had begun, but provided no further details.
A computerized command post exercise, Key Resolve will continue until March 9. Separately, Foal Eagle, a joint air, ground, and naval field training exercise, will be held from March 1 to April 30.
The United States has 28,500 troops stationed in South Korea.
Pyongyang has called the drills a ‘silent declaration of war.’
"Our army and people will foil the moves of the group of traitors to the nation and warmongers at home and abroad for a new war with a sacred war of our own style," the North's National Defense Commission (NDC) said in a statement on Saturday.
It called the two rounds of drills ‘unpardonable infringement upon our sovereignty and dignity.’