South Korean President Lee Jae Myung expressed his regret to North Korea after several unsanctioned drone flights entered the airspace of his secluded northern neighbour.

Lee underlined that the controversial drone flights were carried out by individuals and without the government's approval.

"Even though this was not our government's intention, we express our regret to the North that the irresponsible and reckless actions of a few individuals have caused unnecessary military tensions," Lee said during a Cabinet meeting.

In January, North Korea accused the South of violating North Korea's sovereignty with several drone flights, saying a surveillance drone filmed important facilities in North Korea on January 4 before Pyongyang's military shot down the aircraft.

Three South Koreans were charged in connection with the case, including an intelligence officer and a military officer in late March.

They are accused of having entered North Korean airspace a total of four times since last September using unmanned drones. Their motives remain unclear.

The North Korean leader's sister, Kim Yo Jong, praised the South Korean president's actions as "very fortunate and wise behaviour."

But she also warned that Seoul "should stop any reckless provocation" against Pyongyang and "refrain from any attempt at contact, instead of paying lip-service to the utmost importance of peace and security."

Formally, North and South Korea remain in a state of conflict, as the Korean War from 1950 to 1953 ended only with an armistice, without a peace treaty ever having been signed.