South Korea has fired into North Korea after an "unidentified object" was sent towards the country.
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. It provided no other details.
The mystery flying object may have been a drone, local media speculated.
In 2014, South Korean officials discovered what they described as several North Korean drones that had flown across the border. Those drones were crude and low-tech, but were still considered a potential new security threat.
The Koreas face off across the world's most heavily armed border, and the two sides occasionally clash. In 2014, they traded machine gun and rifle fire after South Korean activists released anti-North Korean propaganda balloons across the Demilitarized Zone that bisects the Korean Peninsula, but no casualties were reported.
Attacks blamed on North Korea in 2010 killed 50 South Koreans.