More than 300,000 South Korean and 15,000 US troops will take part in the drills, Yonhap news agency reported, citing a military official.
South Korean defence 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 30 April, are intended to warn North Korea against provocations, Mr Han was reported as saying.
North Korea sees the annual war games as a rehearsal for invasion.
The US and South Korea on Friday also began formal talks on the deployment of the US missile defence 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.
Can South Korea defend itself?
Dealing with the North: Carrots or sticks?
What is the Terminal High Altitude Area Defense System (Thaad)?
1. The enemy launches a missile
2. The Thaad radar system detects the launch, which is relayed to command and control
3. Thaad command and control instructs the launch of an interceptor missile
4. The interceptor missile is fired at the enemy projectile
5. The enemy projectile is destroyed in the terminal phase of flight
The launcher trucks can hold up to eight interceptor missiles.
The US and South Korea are set to begin their largest ever joint military exercises amid high tensions on the Korean peninsula.