The Demilitarised Zone is on the Korean peninsula and is around 160 miles long.
South Korea's military has said it was a flock of birds that prompted it to alert journalists that it had detected an "unidentified object" flying near the border with North Korea.
The South's Joint Chiefs of Staff had said earlier that its radar found "the traces of flight by an unidentified object" over the central portion of the Demilitarised Zone that divides the two Koreas.
News reports said South Korea launched fighter jets and helicopters in response because it believed it could be a North Korean helicopter flying across the border into South Korea.
The Joint Chiefs of Staff said it could only confirm that it deployed unidentified surveillance assets to find out what the object was.
It said it had confirmed that it was a flock of birds flying across the border from north to south.