Seoul says two South Koreans and 15 Russians have been held in North Korea for a week after their boat drifted into North Korean waters.
Seoul's Unification Ministry said Wednesday that the crew members were aboard a Russia-flagged fishing boat when it was detained by North Korea on July 17.
A ministry statement says North Korea hasn't responded South Korea's repeated calls for their repartition.
North Korea is holding six other South Koreans it has detained in recent years for alleged attempts to build underground churches inside the North and other charges.
Ties between the Koreas improved last year after North Korea entered into talks on its nuclear program. But North Korea has reduced its exchanges with South Korea amid a lack of progress in nuclear diplomacy.