SEOUL -- The Republic of Korea (ROK)?on Monday urged the Democratic People's Republic of Korea (DPRK) to set free a student of?ROK's nationality detained in the North for illegal entry.

The DPRK's KCNA news agency reported on Saturday that Joo Won- moon, 21, was arrested by DPRK border guards on April 22 as he illegally crossed its territory from Dandong in China's Liaoning Province.

The student of South Korean nationality, who was given a US green card, had been residing in New Jersey and studying at New York University.

His parents reportedly said they had heard from Joo that he just would go to China for a trip.

ROK's Unification Ministry said in a statement that it was very regrettable for the DPRK to have detained Joo without any advance explanation to the South Korean government and his family.

Seoul urged Pyongyang to release Joo as rapidly as possible and return him to home, calling for the guarantee of his personal safety according to international practices and for the permission of his access to family and attorney.

The ministry also called for the release of three other South Koreans who have been detained in the DPRK.

Kim Jung-wook, a South Korean missionary, was sentenced last year by a DPRK court to hard labor for life after being convicted of espionage and setting up underground churches. He was arrested in October 2013.

In March, the DPRK told a press conference in Pyongyang that Kim Kuk-gi and Choe Chun-gil were arrested on charges of spying activities for South Korea's National Intelligence Service.