South Korea urged North Korea on Monday to release a South Korean college student with a U.S. green card who has been detained for illegal entry.

North Korea announced over the weekend that it has detained a 21-year-old man studying at New York University. He was arrested on April 22 for illegally entering the communist nation through a Chinese border city.

Pyongyang identified him as Joo Won-moon, residing in New Jersey.

“It is deeply regrettable that North Korea is detaining Joo Won-moon, who is a South Korean national, without any explanation to our government and his family,” Lim Byeong-cheol, spokesman at the Ministry of Unification, said in a statement. “The government strongly demands the North immediately release Joo and return him to the arms of his family.”

Lim stressed Pyongyang should guarantee his security and permit consular access in accordance with international law and practice.

The official also called on the North to release three other South Koreans detained there ― Kim Jung-wook, Kim Kuk-gi and Choe Chun-gil.

Kim Jung-wook, a South Korean missionary, has been detained there for more than a year after being sentenced to hard labor for life on charges of spying and setting up underground churches.

In March, the North also said it arrested two South Koreans, claiming they are agents of the South’s state spy agency.

The North allowed the U.S. cable news network CNN to hold separate interviews with the two in Pyongyang on Sunday, in which they said they worked for the National Intelligence Service.

The two South Koreans told CNN on Sunday that they spied for South Korea, while Seoul has flatly rejected the accusations and urged Pyongyang to release them.

Sunday’s interviews as well as the new detention could be an attempt by Pyongyang to pressure South Korea to shift its policy toward the communist nation. The North could also use the detainees as a negotiating chip should inter-Korean talks reopen.

When announcing last month the arrest of the two CNN interviewees ― Kim Kuk-gi and Choe Chun-gil ― North Korea branded them as “heinous terrorists.”

In Sunday’s interviews, both Kim, 61, and Choe, 56, admitted to the charges against them and said they would accept any punishment that was decided. North Korean minders were present during the interviews, CNN said.

The news network also said North Korea is often accused of pushing detainees to make false confessions.

Kim was quoted as telling CNN that he was a missionary struggling financially in northern China when the South’s spy agency recruited him. They offered to pay for information about the North, such as itineraries of visits by leaders to foreign countries and copies of the new North Korean currency for forgery.

He was also quoted as saying he was paid about $500,000 over roughly nine years.

Choe said he was working as a businessman in northern China when he was approached by the South’s spy agency to gather information and materials from North Korea. He said he worked as a spy for three years before he was arrested.

They said they were being treated well by the North Koreans, accusing the South of disowning them.

Choe apologized to his family for getting into trouble. Kim said he had no family, but warned other South Koreans against doing as he had done. He also praised North Korean leader Kim Jong-un in “glowing terms,” according to CNN.

From news reports