The South Korean Unification Ministry said Friday that North Korea has refused to accept the South Korean government's letter of demand to set free four South Koreans detained in the North, including a New York University student.

Joo Won-moon, a 21-year-old student, has been detained by North Korea since April 22 after illegally entering the communist country. North Korea has also kept in captivity three other South Koreans, including missionary Kim Jung-wook, Yonhap News Agency reported on Friday.

The Unification Ministry said it attempted to send a letter earlier in the day, calling for the North to repatriate the four detainees, but the North refused to accept it.

The ministry said in a statement that it intended to express regret that the North has been silent over the South's repeated demand for the repatriations.

"The South Korean government has called on the North to reveal their whereabouts and health conditions. It has also urged the North to immediately free those detainees," read the statement. "Seoul will make all-out efforts to bring them home as soon as possible."

Missionary Kim, 52, has been held there since October 2013. The North has described him as a spy for South Korea's National Intelligence Service, who sneaked into the country with the purpose of inciting dissent.

The other two detained South Koreans -- Kim Kuk-gi and Choe Chun-gil -- were recently allowed to have separate interviews with CNN, in which they said they worked for Seoul's spy agency. CNN said it hasn't been able to independently verify their accounts.