South Korea's spy agency has sent summons to several members of a radical labor umbrella group over suspicions of their involvement in an alleged espionage case, sources said Tuesday.

According to prosecution and labor group officials, the National Intelligence Service asked about 10 officials of the Federation of Korean Trade Unions to face questioning about whether they were engaged in spying for North Korea through an underground anti-state organization.

A source at the prosecution, however, played down the possibility that the union officials would be booked on espionage charges.

A source at the federation, which is one of the nation's two largest umbrella labor groups, said that the summons was not directly related to the union group.

Last week, South Korea's prosecution arrested five South Koreans, including a chief executive at a local tech firm, known by his surname Kim, on charges of violating the country's National Security Law.

The tech company chief allegedly set up the anti-state organization and collected and delivered information about South Korea for several years to the North, prosecutors said. Kim, who denied the charges, is also accused of contacting spies in Japan and other North Korean agents during his visits to foreign countries.

Sources said the South's intelligence authorities are investigating several figures in academic and political circles on suspicions of espionage. The probe could have significant repercussions as former or incumbent political figures in opposition parties are said to be on the list.

(Yonhap News)