Seoul, Jan 20 (AHN) South Korean President Lee Jae Myung on Tuesday instructed authorities to conduct a thorough investigation into a civilian accused of flying a drone into North Korea, calling it an unacceptable act that requires stern measures.

Lee made the remarks during a Cabinet meeting, as the civilian was questioned by a joint military-police investigation team last Friday over the alleged drone incursion.

"It is unacceptable to send drones to the North for unlawful purposes or for a civilian to infiltrate drones into North Korean territory," he said.

North Korea claimed early this month that South Korea infringed on its sovereignty with drone incursions in September last year and on January 4.

The influential sister of North Korean leader Kim Jong-un demanded Seoul admit to and apologise for the incident.

South Korea's military has denied such claims, saying the drones in question were not models operated by the military, Yonhap news agency reported.

Lee said it was unimaginable that a civilian alone could have sent the drone to gather intelligence from the North, noting speculations that state institutions could have been involved in the case.

"From what has been revealed so far, a civilian recklessly sent a drone to North Korea, which is tantamount to starting a war," he said, likening the act to 'firing a gun' at the North.

"We must investigate the matter thoroughly and take stern measures to ensure such acts never happen again," he added.

Lee then reprimanded Defence Minister Ahn Gyu-back for failing to detect the drone activities.

"How could you not detect the drones moving back and forth despite such developments in technology and defence capabilities," he said.

"This means there is a hole in our surveillance system."

He also instructed the authorities to avoid an unnecessary escalation of tensions with the North, warning that further confrontation could harm the economy and strain inter-Korean ties.