South Korea urged North Korea Monday to release a South Korean college student with a U.S. green card detained in the North for illegal entry.

North Korea announced over the weekend that it has detained a 21-year-old man studying at New York University. The North said it arrested him 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 the other 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 have separate interviews with the two in Pyongyang on Sunday, in which they said they worked for the National Intelligence Service.

CNN said it hasn't been able to independently verify their accounts. (Yonhap)