The exercise follows a joint drill by South Korea’s army and air force on Monday involving F-15 fighter jets and land-based ballistic missiles that simulated an attack on North Korea’s nuclear test site to “strongly warn” Pyongyang.

North Korea claimed on Sunday that it has successfully detonated a hydrogen bomb. Hours before the test, photos emerged showing regime leader Kim Jong Un inspecting what it said is a new thermonuclear warhead in a lab.