South Korea's military fired warning shots on 23 may after an unidentified object was spotted over the border with Pyongyang.
South Korea's Joint Chiefs of Staff said in a statement that the South Korean military bolstered its air surveillance and broadcast a warning to North Korea in response to the object.
The mystery flying object may have been a drone, local media speculated.
Recently, U.S. Secretary of State Rex Tillerson visited the heavily militarized border between the rival Koreas.
North Korea on 21 may tested its most powerful intercontinental ballistic missile (ICBM) on 21 may from a location near Pukchang .
At a time when U.S. President Donald Trump is considering diplomatic and other options including sanctions to counter the threat posed by North Korea's nuclear programme, Pyongyang fired an unidentified projectile showing its defiant behaviour to the international community.