North Korean bombers and fighter jets on Thursday made a rare formation flight and apparently conducted live-fire drills, in an assumed tit-for-tat military action to the recent strike exercises of the air forces of South Korea and the US.

South Korea’s Joint Chiefs of Staff said its military detected four North Korean bombers and eight fighter jets flying in formation over North Hwanghae Province, which borders South Korea.

Twelve North Korean warplanes were seen crossing a “special reconnaissance line” -- arbitrarily designated by the South Korean military -- at around 2 p.m.

The special reconnaissance line is north of the inter-Korean Military Demarcation Line and the South Korean military’s tactical action line. But the South Korean military takes military counteraction in such a case that North Korean aircraft cross the special reconnaissance line.

The formation of bombers and fighters staged a formation flight for around one hour in the area south of the special reconnaissance line.

North Korea’s combat aircraft are “presumed to conduct air-to-surface firing drills” in the area, according to the JCS.

The North Korean combat aircraft conducted aerial live-fire drills while moving from Koksan County  to Hwangju County in North Hwanghae Province, The Korea Herald learned. Koksan and Hwangju are about 65 km and 100 km from the inter-Korean border.

The exact route flown and the location of the reconnaissance line were not disclosed.

The South Korean military responded by immediately scrambling around 30 Air Force fighter jets, including F-15Ks, according to the JCS.

North Korea’s formation flight is largely seen as in protest of the strike drills conducted by the South Korean and US air forces on Tuesday. North Korea had not made such a formation flight south of the special reconnaissance line for at least the past year.

Four South Korean Air Force F-15K fighter jets flew with four F-16 Fighting Falcons from the US Air Force, forming a strike package, Tuesday afternoon in a show of force against North Korea.

An F-15K fighter jet fired two air-to-ground joint direct attack munitions at a target on Jikdo, an island off the west coast, to demonstrate its precision strike capabilities.

The aerial strike drills were conducted hours after North Korea launched a suspected intermediate-range ballistic missile that traveled around 4,500 kilometers and flew over Japan.