Seoul has urged Pyongyang to release four South Koreans who were arrested in North Korea, including two spies and a 21 year old New York university student.
North Korea detained two men who confessed to having shared intelligence with South Korea’s spy agency in an interview with CNN in March, although their confessions have not been independently verified.
The two South Koreans, Kim Kuk Gi and Choe Chun Gil, are accused of working for Seoul's National Intelligence Service in the Chinese border city of Dandon when they were arrested.
North Korean State Media accused one of the two of leaking information on the country using USB sticks and memory cards.
South Korea has called the allegations “groundless.”
Joo Won-moon (21), a US citizen, was detained on April 22 while he was crossing from the Chinese side of the Yalu River, North Korea's KCNA news agency said.
South Korea's Unification Ministry demanded North Korea release Joo.
"The government strongly demands North Korea immediately release Joo Won-moon and return him to his family," spokesman Lim Byeong-chel said
"We strongly point out that as North Korea repeatedly commits anti-humanitarian acts, it will draw stronger criticism from South Koreans and the international community on North Korea's human rights violations," Lim added.
In March South Korea proposed a meeting in which the detention of Choe and Kim could be discussed but North Korea rejected the proposal. It has continued to to search for a diplomatic solution to achieve the release of the tw.
CNN claimed on Saturday that North Korea had a separate meeting with Choe and Kim with official minders present.
Choe confessed that while he was working as a businessman he was also sharing intelligence with the spy agency. Choe was allegedly arrested while trying to obtain boxes of materials from North Korea that were related to military operations, CNN reported.
Kim said he had worked as a spy for nine years. South Korea's National Intelligence Service allegedly wanted him to trace the schedules of foreign leaders who visited North Korea and collect other information.
The two are awaiting trial.
North Korea accuses university student Joo of having also been a spy in recent years.
The Stern School of Business in New York said Joo is their student but he did not take classes this semester. The school added that they have no information about his travels.
North Korea also arrested a South Korean missionary ,Kim Jeong-wook, and sentenced him to life in prison for espionage and setting up an underground church. South Korea has also demanded his release.
North Korea recently released three Americans, including Kenneth Bae, who had been held for two years.