Monday's incursion prompted Seoul to fire warning shots and deploy fighter jets and attack helicopters to shoot down the drones, one of which flew close to the capital.

"Yesterday, five enemy drones invaded South Korean airspace, and our military detected and tracked them, but we apologise for not being able to shoot them down," the Joint Chiefs of Staff (JCS) said in a statement.

All of the drones appeared to have returned to the North despite an operation to hunt them down that lasted five hours.

The JCS acknowledged that, while the military can counter "attack drones that pose a real threat", there is a limit to their ability to detect and strike smaller spy drones.

"As a result, the military's lack of readiness has caused a lot of concern to the public," it said.

South Korean President Yoon Suk-yeol said at a Cabinet meeting on Tuesday the incident showed that the military's readiness was "greatly lacking".

Yoon ordered an acceleration of a plan to launch a drone unit to carry out surveillance and reconnaissance operations on North Korea's key military facilities. The plan includes introducing advanced stealth drones.

The incursion was the first time in years that North Korean drones had entered the South's airspace and closely followed a recent flurry of sanctions-busting weapons tests by Pyongyang.