South Koreans charged with spying for North Korea met secretly with North Korean agents in Cambodia and other Southeast Asian countries, according to Seoul prosecutors.

The Seoul Central District Prosecutors’ Office wrote in an indictment released late Thursday that the head of the alleged spy ring, a 60-year-old identified only by his surname of Hwang, made overseas trips to communicate with North Korean agents in person.

In March 2016, in one of the earliest of such encounters, Hwang stayed at a resort in Cambodia where he and two North Korean agents met up and exchanged intelligence, prosecutors said.

Prosecutors said Hwang eventually made other members of the spy ring, named the People’s Unification Front, travel to countries like Cambodia and Vietnam to meet North Korean agents there.

Hwang gave the spy ring members specific instructions on how and where to find the North Korean agents, and cautioned them about being followed or watched.

One of them, a 44-year-old woman surnamed Jung, gave the North Korean agents her personal details such as her hometown and the identities of her children to gain their trust, prosecutors said. She even submitted a written pledge of loyalty to North Korean leader Kim Jong-un.

At one point Jung received about $7,000 from North Korea for her part in aiding their spying operations, prosecutors found.

In a different investigation into alleged spies for North Korea, prosecutors on Thursday filed a request for a warrant to arrest four officials of South Korea’s umbrella labor union organization, the Korean Confederation of Trade Unions.

The KCTU officials met with North Korean authorities overseas and communicated with them over several years, prosecutors said.

Prosecutors also claim North Korea influenced the umbrella union's protests against the South Korea-US joint military exercises, allegedly orchestrating the protests and sending specific protest slogans.