South Korean activists have been charged with espionage for allegedly taking orders from North Korea to stage protests opposing Seoul's plan to bring in stealth fighter jets from the U.S.

According to Yonhap news agency on Saturday, the National Intelligence Service (NIS) and the police obtained USB flash drives while searching the homes of four activists based in Cheongju, North Chungcheong Province in May, and found some 80 orders and reports in the form of encrypted files exchanged with North Korean spies since 2017.

The suspects, three of whom have been arrested, set up an organization and received orders from the North to recruit around 60 politicians and labor and civic group officials in the Chungcheong area to stage protests opposing the introduction of U.S. stealth fighter jets.

The North Korean spies are members of the regime's anti-South propaganda organ called the United Front Department.

The NIS and police have charged the suspects with violating the National Security Law. They are believed to have received 20-thousand dollars in operating funds from the North.