An aircraft presumed to be a North Korean helicopter was observed flying south near the North Korean border city of Gaeseong Thursday, prompting a responsive deployment of South Korean fighter jets. 

A South Korean military source said the North Korean aircraft crossed the tactical action line, an imaginary air boundary 20 kilometers to 50 kilometers away from the Military Demarcation Line(MDL) and the de-facto maritime border, Northern Limit Line(NLL). 

It did not, however, approach the no-fly zone set up within ten kilometers of the MDL. 

It was the second such case in five days and the South Korean military authorities are not ruling out the possibility they are related with a seasonal military drill by the North or a field trip by a senior Pyongyang official to a military unit near the border.