South Korea has told Taleban militants there is a limit to what it can do to meet their demands for the release of 21 South Korean hostages.

South Korean presidential spokesman Cheon Ho-Seon says during contact with the kidnappers, officials have made clear the South Korean government is not in a position to answer Taleban demands to free militant prisoners held by the Afghan government.

The two sides are trying to agree on a place for face-to-face talks.  The Taleban has said it prefers to meet in Afghan territory under its control. 

Meanwhile, a team of private Afghan doctors has volunteered to treat the remaining hostages, two of whom are reported to be seriously ill.  It is not clear if the offer has been accepted.

The kidnappers have already killed two male hostages, accusing the Afghan government of ignoring their demand to release rebel prisoners.

The remaining hostages include 18 women and three men, all of them South Korean Christians on a volunteer humanitarian mission.

The Afghan government has refused to negotiate a prisoner swap since July 19 when the 23 Koreans were kidnapped while on a bus headed to Kandahar from Kabul.