His comments come as Taliban militants threatened to kill a group of South Korean hostages today unless Seoul withdraws its some 200 troops from Afghanistan.
Song said the withdrawal at the end of the year was scheduled.
Afghan police say Taliban fighters kidnapped the 18 South Koreans from a passenger bus on July 19. The group has been identified as members of a Christian evangelical organization.
Ghazni Province deputy police chief Mohammad Zaman says gunmen stopped the bus on the country's main highway from Kabul to Kandahar.
A purported Taliban spokesman, Qari Mohammad Yousuf Ahmadi, says 18 South Koreans were seized -- including 15 women and three men. He said their activities are being "investigated" and the Taliban will announce its demands later.
A senior administrator at the Saem-Mul Protestant Community Church near Seoul told AFP that the hostages are from the church. The administrator said the group has been on an evangelical mission in Afghanistan since July 13 and was due to return to South Korea on July 23.