South Korea has urged Pyongyang to release four of its citizens being held by the North, including two men who told CNN they spied for the South, and a 21-year-old New York University student.

Two men arrested by North Korea in March said in interviews with CNN they spied for South Korea's intelligence agency.

North Korea said Kim Kuk Gi and Choe Chun Gil were South Korean nationals working as spies for Seoul's National Intelligence Service from the Chinese border city of Dandong.

North Korean state media accused one of them of running an "underground church" and spreading foreign information on USB sticks and SD memory cards in the country.

South Korea has called the allegations "groundless".

Pyongyang said it had arrested a South Korean man with a US green card who was a student at New York University. Joo Won-moon, 21, was detained on April 22 crossing from the Chinese side of the Yalu River, according to North Korea.

"As North Korea repeatedly commits anti-humanitarian acts, it will draw stronger criticism from South Koreans and the international community," South Korean Unification Ministry spokesman Lim Byeong-cheol said.

In late March, the South sent a message to the isolated North proposing a meeting to discuss the detentions of Mr Choe and Mr Kim but the North declined to accept it, Mr Lim said. He said South Korea had been seeking diplomatic help from countries that have embassies in both countries.