South Korea has fired warning shots at an unidentified object flying south from North Korea, its military said.
Local media said the object might have been a North Korean drone.
The Joint Chiefs of Staff said in a statement that the military bolstered its air surveillance and broadcast a warning to North Korea in response to the object.
South Korea's Yonhap news agency, without citing a source, reported that the South fired about 90 machine gun rounds into the air and towards North Korea.
It said officials are analysing whether a North Korean drone had crossed the border.
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-Pyongyang propaganda balloons across the Demilitarised Zone that bisects the Korean peninsula, but no casualties were reported.
Attacks blamed on North Korea in 2010 killed 50 South Koreans.