The Ulchi Freedom Guardian, which will run through August 28th, is aimed at helping the allies better react to the wide range of threats posed by Pyongyang.
The drills involve about 50-thousand South Korean and 30-thousand American troops in a wargame that simulates a North Korean invasion.
Officials say Pyongyang was informed of the allies' plans on Saturday through loudspeakers at Panmunjom, the truce village inside the DMZ.
North Korea has a history of condemning the drills, calling them practice for an invasion of the North.
This year is no different as Pyongyang threatened over the weekend to take strong military counteractions against the U.S. if the drills proceed.
"In response to the Ulchi Freedom Guardian exercise, we will take intense and appropriate counter-attack measures."
North Korea has also resumed anti-Seoul broadcasts at the border, further heightening tensions between the two Koreas.
The move is seen as a response to South Korea bringing back anti-Pyongyang broadcasts. The South started sending messages across the border with loudspeakers after two South Korean soldiers were injured by landmines in the DMZ.
A South Korean military official says the North's broadcasts are aimed at preventing its people from hearing the South's anti-Pyongyang broadcast, rather than conveying messages across the border.
South Korea's defense ministry says it has strengthened its security level in case of any additional provocations from the North.
Kim Hyun-bin, Arirang News.