The main opposition People Power Party (PPP) called on the defense minister to step down Friday, urging him to take responsibility for the military mistaking a U.S. spy drone near the inter-Korean border as an enemy threat.

PPP chief spokesperson Park Choong-kwon, a North Korean defector-turned-lawmaker, made the call in a statement, insisting the incident showed a serious lapse in military discipline.

"After shocking the public by leaving front-line general outposts and guard posts without live ammunition, the military has now committed another unprecedented security failure by failing to identify an allied drone and preparing to shoot it down," he said.

"The PPP will hold those responsible for this incident accountable for undermining the military's surveillance and reporting systems," he added.

He then urged Defense Minister Ahn Gyu-back to resign immediately for driving the national security into a state of "total collapse."

The unmanned flying vehicle was spotted the previous day in the buffer zone, known as the Civilian Control Line, south of the Military Demarcation Line 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 to identify the drone.

The South's Joint Chiefs of Staff later confirmed it was a U.S. military drone belonging to the U.S. Marine Corps, which was carrying out an operation during a combined exercise with South Korean Marines near the border.