South Korea's spy agency said Monday it has detained three more people affiliated with a leftist party for allegedly plotting to topple the government by destroying national infrastructure in support of North Korea in case of an inter-Korean war.

The detention of Kim Hong-ryeol, the head of the Gyeonggi Province branch of the United Progresive Party; Kim Geun-rae, the branch's deputy head; and Cho Yang-won, a think tank head, comes after the prosecution indicted with physical detention UPP Rep. Lee Seok-ki and three other senior party members last week on similar charges.

North Korea threatened nuclear war with South Korea and the United States early this year when the United Nations imposed stricter sanctions on the North after Pyongyang's third nuclear test in February.

North Korea recently cancelled a planned reunion of families separated by the 1950-53 Korean War, citing what it called the "suppresion of pro-reunification patriots," apparently referring to Rep. Lee and other UPP members.

The National Intelligence Service accused the three of alleged treason under the Criminal Code and of praising and sympathizing with the enemy under the anti-communist National Security Law.

The accused are suspected of discussing with other members of the underground entity called the Revolution Organization at a meeting in Seoul in May ways to destroy communication lines, railways and major oil tanks in support of North Korea if war broke out.

The three are also charged with making remarks siding with North Korea and singing North Korean revolutionary songs at several meetings held between March and August.

The Suwon District Court will interrogate the three people Tuesday to review the legality of their detention, according to court officials. (Yonhap News)