Drones sent into North Korea earlier this year were initially denied by Seoul.
President Lee described the actions as 'unnecessary' and 'irresponsible.'
A probe revealed government officials' involvement in the drone incidents.
Briefing: South Korea's president regrets 'reckless' drones sent into the North
Strategic angle: President Lee Jae Myung expresses regret over the government's involvement in drone incidents with North Korea.