The United States and South Korea began the annual military exercises on Monday despite ongoing threats from North Korea, the Yonhap news agency said.
The drills, headlined The Key Resolve, will continue until March 9. It will involve about 200,000 South Korean troops and 2,100 U.S. servicemen. Australia, Canada, Denmark, Norway and Britain will take part in the Key Resolve as observers.
Separately from the Key Resolve, another joint exercise, the Foal Eagle, will take place from March 1 to April 30, involving about 11,000 U.S. forces plus South Korean troops.
Over the weekend Pyongyang warned it was ready to launch a "sacred war" against South Korea and the U.S.
On Monday, North Korea accused the two allies of being "poised for a war carrying nuclear war equipment" and dubbed the South and the U.S. "warmongers."
Pyongyang also described the exercises as "an unpardonable infringement upon the sovereignty and dignity of the DPRK” saying they target the communist state during its mourning period for the late leader Kim Jong-il.