Prosecutors said on Thursday they have investigated 10 South Koreans of spying for the North Korean regime for more than a decade, leaking political and military secrets in the latest in a long series of espionage cases.

Operating in an underground anti-state organization called Wangjaesan, the name of a North Korean mountain, they allegedly dug up current political and military information and reported it to the North Korean intelligence agency, the Seoul District Prosecutors' Office said.

Out of the 10 under investigation, prosecutors indicted five including a 48-year-old man surnamed Kim, who acted as the ringleader, on charges of forming the spy group and engaging in espionage in violation of the National Security Law. The five others are currently being questioned without physical detention, the prosecution said.

The latest crackdown came after Han Sang-dae, South Korea's new prosecutor general, declared war on pro-North Korean activities and corruption in his inaugural address on Aug. 12. Han promised to reinforce the prosecution's public security organizations in order to more actively conduct investigations into pro-North Korean forces here.

According to the prosecutors' office, Kim, one of the accused, is believed to have met in person with North Korean founder Kim Il-sung in August 1993 after the communist country's intelligence agency known as Unit 255 recruited him a few years earlier.

At the meeting, Kim received an order from North Korea's "eternal president" to establish a South Korean unit that would assist the North in its attempts to "revolutionize" the South. The goal was to disseminate and instill communist ideas among South Koreans, the prosecution said.

Back in the South, Kim successfully employed his schoolmates to become North Korean spies before finally launching the underground organization in March 2001, the office noted.

The organization then gathered information for Pyongyang on the latest South Korean political affairs, key U.S. military bases in the South as well as military strategies, the office noted. One of the five indicted even found employment as a secretary for former liberal party lawmaker Lim Chae-jung, who chaired the National Assembly from 2006-08.

The organization leader financed the espionage activities using proceeds from a firm that he established that provided historical content, as well as two other related companies he set up to secure funds.

The five indicted people were also found to have been decorated by the North regime for their contributions to the communist country's intelligence work.

Kim had been in frequent contact with North Korean agents over the years, mostly outside South Korea. He received orders on 34 occasions by making secret contact with North Koreans, mostly in China, Japan and Malaysia, the prosecution added.

The North Korean spy unit is believed to have directed its South Korean operatives to take over a public administrative building, military camps and a broadcasting station in Incheon "in the event of an emergency" as part of its strategy to control the city that borders the South's capital as well as North Korea.

The latest anti-state charges follow many similar espionage cases in the past in which North Korea tapped South Korean agents since the two countries were divided following the 1950-53 Korean War that ended in a truce, not a peace treaty. (Yonhap News)