South Korean military officials say a group of 12 North Korean warplanes has flown in formation over a southwestern part of North Korea and presumably held firing exercises.

The officials say eight fighter jets and four bombers flew over North Hwanghae Province for more than one hour from around 2 p.m. on Thursday. They say the planes are thought to have staged air-to-surface firing drills.

The officials say the aircraft crossed the so-called special reconnaissance line. The South Korean military set the special reconnaissance line north of the inter-Korean military demarcation line to monitor the activities of North Korean warplanes.

The officials say the military scrambled some 30 jets, including F-15Ks, on a surveillance mission in response.

They add it is rare for North Korean military planes to carry out a formation flight south of the reconnaissance line.

The flight follows North Korea's latest missile launch. The country fired two short-range ballistic missiles from the Samsok area in Pyongyang toward the Sea of Japan Thursday morning.

South Korean media report the North Korean flight may have been in protest of a joint drill conducted by Japan, the United States and South Korea in the Sea of Japan on Thursday. The exercise simulated how to respond to ballistic missile launches.