The United States and South Korea kicked off their annual summertime joint military exercise on Monday, amid efforts by Seoul to improve relations with North Korea.

Ulchi Freedom Shield will involve 21,000 soldiers in computer-simulated command post operations and field training.

In a speech to mark the 80th anniversary of Korea's liberation from Japanese colonial rule, Lee said he would restore the so-called September 19 Comprehensive Military Agreement, a de-escalation measure that halted some military activities at the border between North Korea and South Korea.