The South Korean military detected an unidentified projectile crossing the military demarcation line in Cheorwon, Gangwon province at about 4 p.m. local time (0700 GMT), according to the Joint Chiefs of Staff (JCS).
The Seoul military conducted a warning broadcast and fired warning shots toward the flying object in accordance with proper procedures, the JCS said in a statement.
The flying object returned back to the DPRK, and the South Korean military was analyzing the projectile.
The statement said the military strengthened its air observation and maintained defense readiness.
