Four South Korean activists have been charged with espionage for allegedly taking orders from Pyongyang and staging a series of protests opposing Seoul's plan to procure US-built stealth fighter jets, sources said Friday.

According to the sources, the National Intelligence Service (NIS) and the National Police Agency (NPA) recently requested a local court to issue warrants to arrest the activists who allegedly violated multiple articles of the National Security Act, including the so-called espionage article.

The district court in Cheongju, about 140 kilometers south of Seoul, allowed the authorities on Monday to arrest three of the activists, citing flight risk.

The article stipulates the death penalty or life imprisonment for those who leak military or state secrets that are accessible to limited persons to a hostile country or anti-government organization. Those who share intelligence of lesser importance can be sentenced to seven years or longer in prison.

The activists have been probed by the NIS and NPA on charges of receiving orders from North Korea to hold rallies and petition campaigns opposing the South Korean military's plan to purchase American F-35A stealth fighters.

They allegedly have been meeting North Korean spies in China and Cambodia since 2017, and received $20,000 from the agents to form an underground organization supporting Pyongyang in South Korea.

In May, the NIS and NPA raided the activists' residences and offices in Cheongju, and seized USB flash drives that contained documents they exchanged with North Korea.

They also reportedly met with an unidentified senior lawmaker from the ruling Democratic Party to propose creating inter-Korean businesses.

One of the activists was found to be the CEO of a local online newspaper and another a worker who had been dismissed by a major South Korean company.

The NIS investigated them for years and asked for police cooperation earlier this year. (Yonhap)