North Korea will return six unidentified South Koreans across the two countries' heavily-militarised land border, it said in a surprise announcement Thursday.
The South's Unification Ministry said it had been informed the transfer would take place on Friday, but was given no details of the identities of the people being returned.
"We just received a sudden notice from North Korea that it would send back six South Koreans tomorrow," a ministry official told AFP.
The repatriation will be made through the truce village of Panmunjom, where the armistice ending hostilities in the 1950-53 Korean War was signed.
In a statement, the Unification Ministry noted that North Korea had announced the detention of four South Koreans in February 2010, and that these might be among the six being returned.
"Our government had repeatedly asked for their identities, but never received an answer," the statement said.
While more than 23,500 North Koreans have escaped to the South since the end of the Korean War, the number of defectors going the other way has been miniscule.
In an extremely rare case last month, South Korean border guards shot dead a man as he tried to swim across the border to North Korea in broad daylight.
South-North relations have been on a rollercoaster ride this year, following the North's third nuclear test in February which triggered an extended spike in military tensions.
By the end of August, the situation had calmed down and the two sides seemed to be drawing closer, with agreements on a series of key cross-border projects.
But the mood soured again last month after Pyongyang cancelled a scheduled reunion for family members separated by the war.