They will run until Friday at the risk of upsetting the North, which has often used allied drills as a pretext for its own ballistic missile tests -- even though Pyongyang is barred from such technology by the United Nations.
According to local news agency Yonhap, powerful assets involved in the exercises include the USS Ronald Reagan aircraft carrier strike group along with U.S. special forces aboard a nuclear-powered submarine.
Elsewhere in the South, a U.S. Air Force F-22 Raptor fighter jet normally based in Alaska roared over the peninsula for a rehearsal flight ahead of the Seoul International Aerospace and Defense Exhibition starting Tuesday.
"We are here fully supporting the ironclad alliance and showing our support, bringing the F22 Raptor and showing the capabilities of the aircraft," Maj. Dan Dickinson, commander of U.S. Air Force's F-22 Demo Team, was quoted by Yonhap as telling reporters.
"The capabilities of the jet really exceed anything else you've seen in the world today," he added.
The U.S. is also set to demonstrate its F-35A stealth fighter for the first time in South Korea.
In addition, the show will feature a B-1B Lancer strategic bomber -- the same model that flew close to North Korea last month, provoking Pyongyang to threaten to shoot down American aircraft.