A mix-up over a U.S. spy drone flying near the inter-Korean border has raised questions about a potential communication gap between the allies after Korean forces mistook the drone for an enemy threat and had almost prepared to shoot it down.

On Thursday, the unmanned flying vehicle was spotted in the buffer zone, known as the Civilian Control Line, south of the Military Demarcation Line (MDL) separating the two Koreas.

The Army's 1st Corps, responsible for front-line defense, activated Durumi, an anti-air defense alert system issued in the event of an enemy drone incursion, and used surveillance equipment, including thermal observation devices (TOD) and radar, to identify the drone.

Korea's Joint Chiefs of Staff (JCS) later said it was a U.S. military drone belonging to its Marine Corps carrying out an operation during a combined exercise with the Korean Marines near the border.

But questions have arisen over why the Korean Army was not aware of the allies' drone operation in the first place. The Army's Ground Operations Command had reportedly mobilized its air defense assets, such as anti-aircraft artillery, to down the drone.

The U.S. Forces Korea (USFK) reportedly gave prior notice regarding its plan to operate drones near the border, but the Korean military claims it never granted formal approval.

The Chosun Ilbo daily reported Thursday that the USFK informed the Korean side of the plan to hold the drills and sent an official document regarding the drone operation, but it was never reported up the chain of command or shared with relevant units, citing unidentified military sources.

Under the Korean military protocol, prior notification must be submitted to the Ground Operations Command for approval.

The JCS said it is gathering facts regarding the reporting chain and trying to determine whether the U.S. drone flew with proper authorization.

The USFK said earlier that it is assessing the circumstances surrounding the incident and will continue working with the Korean military and relevant authorities on the matter.