South Korea on Wednesday welcomed the release of an American college student long detained in North Korea and vowed efforts to free its own citizens held in the communist country.

The US Department of State announced that Otto Warmbier, a 22-year-old University of Virginia student, was released after 17 months of detention.

He was arrested in January last year for allegedly stealing a political propaganda sign from a hotel in North Korea and was later sentenced to 15 years of hard labor. 

"The release is very welcome. ... We expect there will be a detailed description of the release through the diplomatic channel (with the US)," Lee Duk-haeng, spokesman at the Ministry of Unification, said in a press briefing.

Lee said South Korea is also making efforts to seek the release of its own citizens now held in North Korea. "Since we also have our citizens detained in the North... we are putting pressure on the North to free them through various channels," he said, vowing "utmost efforts."

Currently, a total of six South Koreans are detained in North Korea, including several religious missionaries accused of treason or espionage.

North Korea has a track record of detaining South Korean and American travelers, and using them as diplomatic leverage in negotiations with the detainees' home countries.

Meanwhile, the ministry said that it has approved three more requests by local civic groups to contact North Koreans, raising the total to 21.

Seoul said last month that it will flexibly resume civilian inter-Korean exchanges to an extent that the move does not compromise the international sanctions regime.

But North Korea has so far rejected South Korean civic groups' bids to visit the North for exchanges, taking issue with South Korea's support of the latest UN sanctions. (Yonhap)