More than 300,000 South Korean and 15,000 U.S. troops will take part in the drills, Yonhap news agency reported, citing a military official, the BBC reported.
South Korean defense minister Han Min-koo has said the exercises will be twice the size of last year.
The drills start days after the UN passed new sanctions on North Korea.
Security tensions have increased since the North tested a nuclear device in January, followed by a rocket launch.
The North responded to the sanctions by saying it was readying nuclear weapons for 'pre-emptive' use, and firing short-range missiles into the sea.
Military analysts doubt the country has the ability to put nuclear warheads on its missiles.
The exercises, which begin on Monday and run until April 30, are intended to warn North Korea against provocations, Mr. Min-koo was reported as saying.
North Korea sees the annual war games as a rehearsal for invasion.
The U.S. and South Korea on Friday also began formal talks on the deployment of the U.S. missile defense system to the peninsula, a move strongly opposed by North Korea, Russia and China.
Beijing says the Thaad anti-missile system compromises its security and would undermine its nuclear deterrent.
