North Korea said earlier this month that a drone from South Korea was flown into its airspace recently, showing photographs of debris and aerial pictures of buildings that it alleged the drone had taken.

A South Korean task force investigating the incident has called in a civilian suspect for questioning, police said.

“It should not happen for a civilian to send a drone to North Korea for an illegal purpose,” Lee told a televised cabinet meeting. “How did we fail to check that a drone had been flown to the North?”

Lee said such action was “akin to starting a war” and created unnecessary tension with North Korea that could have a negative impact on the South Korean economy. He called for a thorough investigation and to ensure it never happened again.

Lee said that while a probe was ongoing if a South Korean citizen was found to have flown the drone they could be criminally charged for provoking Pyongyang.

Kim Yo Jong, the powerful sister of North Korean leader Kim Jong Un, last week urged Seoul to investigate the incident, warning that any provocations could result in terrible situations.

Lee’s administration has been seeking to improve ties with Pyongyang since he took office last year, but so far its overtures have been rebuffed by the North.