South Korean army's K-55 self-propelled artillery vehicle moves during a military exercise near the demilitarized zone between the two Koreas in Cheorwon, South Korea, Monday, Aug. 21, 2017. U.S. and South Korean troops kicked off their annual drills Monday that come after U.S. President Donald Trump and North Korea exchanged warlike rhetoric in the wake of the North's two intercontinental ballistic missile tests last month. (AP Photo/Ahn Young-joon)

South Korean police officers aim their machine guns during an anti-terror drill as part of Ulchi Freedom Guardian exercise, in Goyang, South Korea, Monday, Aug. 21, 2017. U.S. and South Korean troops kicked off their annual drills Monday that come after U.S. President Donald Trump and North Korea exchanged warlike rhetoric in the wake of the North's two intercontinental ballistic missile tests last month. (AP Photo/Ahn Young-joon)

South Korean police officers conduct an anti-terror drill as part of Ulchi Freedom Guardian exercise, in Goyang, South Korea, Monday, Aug. 21, 2017. U.S. and South Korean troops kicked off their annual drills Monday that come after U.S. President Donald Trump and North Korea exchanged warlike rhetoric in the wake of the North's two intercontinental ballistic missile tests last month. (AP Photo/Ahn Young-joon)

A South Korean police officer aims his gun during an anti-terror drill as part of Ulchi Freedom Guardian exercise, in Goyang, South Korea, Monday, Aug. 21, 2017. U.S. and South Korean troops kicked off their annual drills Monday that come after U.S. President Donald Trump and North Korea exchanged warlike rhetoric in the wake of the North's two intercontinental ballistic missile tests last month. (AP Photo/Ahn Young-joon)

Mock terrorists threaten hostages during an anti-terror drill as part of Ulchi Freedom Guardian exercise, in Goyang, South Korea, Monday, Aug. 21, 2017. U.S. and South Korean troops kicked off their annual drills Monday that come after U.S. President Donald Trump and North Korea exchanged warlike rhetoric in the wake of the North's two intercontinental ballistic missile tests last month. (AP Photo/Ahn Young-joon)

South Korean firefighters wearing protective gears attend an anti-terror drill as part of Ulchi Freedom Guardian exercise, in Goyang, South Korea, Monday, Aug. 21, 2017. U.S. and South Korean troops kicked off their annual drills Monday that come after U.S. President Donald Trump and North Korea exchanged warlike rhetoric in the wake of the North's two intercontinental ballistic missile tests last month. (AP Photo/Ahn Young-joon)

South Korean government officials wearing gas masks attend an anti-terror drill as part of Ulchi Freedom Guardian exercise, in Goyang, South Korea, Monday, Aug. 21, 2017. U.S. and South Korean troops kicked off their annual drills Monday that come after U.S. President Donald Trump and North Korea exchanged warlike rhetoric in the wake of the North's two intercontinental ballistic missile tests last month. (AP Photo/Ahn Young-joon)

South Korean police officers rappel down from a helicopter during an anti-terror drill as part of Ulchi Freedom Guardian exercise, in Goyang, South Korea, Monday, Aug. 21, 2017. U.S. and South Korean troops kicked off their annual drills Monday that come after U.S. President Donald Trump and North Korea exchanged warlike rhetoric in the wake of the North's two intercontinental ballistic missile tests last month. (AP Photo/Ahn Young-joon)

A U.S. Air Force U-2 spy plane takes off on the runway at the Osan U.S. Air Base in Pyeongtaek, South Korea, Monday, Aug. 21, 2017. U.S. and South Korean troops kicked off their annual drills Monday that come after U.S. President Donald Trump and North Korea exchanged warlike rhetoric in the wake of the North's two intercontinental ballistic missile tests last month. (Lee Sang-hack/Yonhap via AP)

A U.S. Air Force U-2 spy plane takes off on the runway at the Osan U.S. Air Base in Pyeongtaek, South Korea, Monday, Aug. 21, 2017. U.S. and South Korean troops kicked off their annual drills Monday that come after U.S. President Donald Trump and North Korea exchanged warlike rhetoric in the wake of the North's two intercontinental ballistic missile tests last month. (Lee Sang-hack/Yonhap via AP)

A U.S. Air Force U-2 spy plane prepares to land at the Osan U.S. Air Base in Pyeongtaek, South Korea, Monday, Aug. 21, 2017. U.S. and South Korean troops kicked off their annual drills Monday that come after U.S. President Donald Trump and North Korea exchanged warlike rhetoric in the wake of the North's two intercontinental ballistic missile tests last month. (Lee Sang-hack/Yonhap via AP)