The men are accused of working for an underground spying group called Wangjaesan, the name of a North Korean mountain, under the direction of a man called Kim, a report by the Yonhap news agency said.
Five other men are under investigation as part of the security authority's crackdown on the group that operated allegedly for a decade out of Seoul passing military and political information to North Korea. But the five men aren't under detention, Yonhap reported.
The indictments come after a report by Yonhap earlier this month that said "several members of a radical labor umbrella group," the Federation of Korean Trade Unions, were being questioned over suspected involvement in an espionage case.
A report at the same time by the Web site IntelligenceNews said Seoul's political establishment "has been rocked by the espionage scandal, which allegedly involves several trade unionists, academics and at least 10 members of the country's opposition Democratic Party."
IntelligenceNews said aside from Kim, those being questioned are senior members of the FTA, several academics and at least a dozen opposition political figures.
"Among the latter are members of South Korea's left wing Democratic Labor Party, widely considered as the political wing for the Korean Confederation of Trade Unions," the report said.
The latest reports on the indictments said the driving force behind Wangjaesan, Kim, 48, reportedly was recruited in the late 1980s or early 1990s and secretly met North Korea's founder Kim Il Sung in August 1993.
In Seoul, Kim alleged set up Wangjaesan in 2001, supposedly recruiting people he knew from school.
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 said.
One of the five indicted is a former secretary for the Liberal Party Member of Parliament Lim Chae-jung, who was chairman of the National Assembly from 2006-08, Yonhap reported.
Also, the five indicted men allegedly were decorated by North Korea for their espionage work.
Prosecutors say the five men passed on satellite images of major military installations, field manuals of U.S. forces in South Korea and information on politicians from several parties.
Some of the information they passed to the North concerned South Korea's main airport at Incheon, on an island around 45 miles from Seoul, and the surrounding area.
The airport is strategically important to South Korea's economy. Construction of Incheon airport began in November 1992 on reclaimed land between Yeongjong Island and Yongyu Island. It took eight years to build followed by six months of testing the facility before opening in March 2001.
It is one of the world's busiest airports, and most efficient, for cargo movements, the membership organization Airports Council International said.
The Yonhap agency report gave no details of Kim's education, family connections and work, except that he financed the spy ring using proceeds from his own local technology business and two other companies, also not named.
This month Han Sang-dae, the newly appointed prosecutor general, declared a war on pro-North Korean activities and corruption.
"The prosecution is the guardian of free democracy," Han said in his inaugural address. "It is the prosecution's obligation to crack down on North Korean followers and supporters operating in our society."