A South Korean delegation was in Ghazni province, southwest of Kabul, where the church volunteers were snatched, seeking direct talks with the kidnappers.
But Seoul has told the insurgents there is a limit to what it can do since it has no power to concede the main Taliban demand for the release of rebel prisoners in Afghan jails.
“The venue has not been chosen for talks. Talks are going on to find an agreement on location,” Ghazni police chief Ali Shah Ahmadzai told reporters.
“We are in favour of dialogue, that’s what logic requires. If that doesn’t work, then force may be used,” he said. “If the Taliban do not accept dialogue, that means they do not want this issue to be resolved peacefully.”
There has been a build-up Afghan forces in the Ghazni since the hostages were hauled off a bus on the main road south from Kabul on July 20, but a rescue bid would be fraught with danger.
The Taliban want to hold negotiations in an area they control, and vouched for the safety of the Korean delegates, a Taliban spokesman said on Friday. Otherwise, the insurgents needed United Nations security guarantees should the Koreans want negotiations to take place outside Taliban-controlled areas.
A UN spokesman in Kabul said the world body had yet to receive any request for assistance in holding the talks.
Medical help
Police chief Ahmadzai said authorities had managed to send medicines to the 18 women and three men held by the Taliban in small groups at different locations in Ghazni province.
But the Taliban had rejected a request from a group of private Afghan doctors to visit the captives, Ahmadzai said.
The Taliban have said two of the women are seriously ill.
The kidnappers have killed two of their male hostages, accusing the Kabul government of failing to negotiate in good faith and ignoring their demand to release rebel prisoners.
Afghan officials have refused to free Taliban prisoners, saying that would only encourage more kidnappings.
South Korea has appealed to the United States, which has more than 20,000 troops in Afghanistan, to help free the hostages. Washington has said it would do what it could, but has ruled out making concessions to those it considers "terrorists".