Some in South Korea want the US to intervene in the crisis
A suitable location was believed to be still under discussion after the Taleban rejected a venue under the control of Nato-led forces.
Two of the Christian workers have already been killed.
The Taleban are threatening more deaths unless demands are met for the release of eight government-held prisoners.
The militant group agreed on Thursday to direct talks with a South Korean delegation, which is in Ghazni province where the kidnapping took place.
Seoul confirmed there was contact with the Taleban, without going into details, and warned that there were limits to what it could do to resolve the crisis.
HOSTAGE TIMELINE 19 July: 23 South Korean Christian aid workers seized on bus in Ghazni province 26 July: Body of hostage Pastor Bae Hyung-kyu is found 31 July: Second hostage Shim Sung-min, 29, found shot dead
"Through the direct contacts, we intend to stress that our capabilities to meet Taleban demands are limited," presidential spokesman Cheon Ho-seon told reporters.
His comments appeared to contradict a Taleban spokesman who said on Thursday that Seoul had given assurances that the hostage-for-prisoners swap would go ahead.
Afghan President Hamid Karzai has exchanged prisoners for hostages before, but the government has made it clear this is no longer the policy.
Military build-up
Separately, a group of senior South Korean lawmakers has met senior State Department officials in Washington to lobby for support over the crisis.
Washington has been criticised by some in South Korea for not doing enough to resolve the crisis.
The Seoul government has asked the US and Afghanistan - whose leaders meet this weekend for talks which are expected to include the hostage situation - to use "flexibility" in handling the crisis.
"The Afghan and US governments... have a certain level of involvement in this issue," Cheon Ho-sun said.
"We have expectations that the two leaders would have sufficient understanding of our position when they hold a summit".
It is thought the South Korean aid workers are being held in a number of small groups in a village about 10km (six miles) from Ghazni town centre.
The BBC's Alastair Leithead in Ghazni province said on Thursday that military forces have surrounded the area, searching nearby houses and sealing off roads to the area.
Leaflets were dropped on Wednesday warning local people to leave the area, fuelling rumours that a military rescue operation could be launched.
The South Korean government has repeatedly said it is opposed to direct military action.
However, a senior US diplomat in the region, Richard Boucher, appeared not to rule it out, saying "potential military pressures" were among the "many tools" available to pressure the Taleban.
The Korean Christian workers - 18 of them women - were seized on 19 July as they travelled by bus down the main road from Kabul to Kandahar.
The aid workers' leader, Pastor Bae Hyung-kyu, was the first to be shot dead by the militants. His death was reported over the weekend.
On Tuesday, South Korea confirmed that a second hostage had been killed - 29-year-old Shim Sung-min, a former IT worker.
At least two of the remaining hostages are seriously ill, according to South Korea's Yonhap news agency.
El Talibán y Corea del Sur se han dispuesto a discutir sobre el futuro de los 21 rehenes surcoreanos aún cautivos por el grupo insurgente Talibán, luego de haber sido secuestrados por más 15 días y de haber ejecutado a dos. Una delegación de Surcorea ha sido enviada a la provincia de Gazni (140 km al sur de Kabul), donde fueron secuestrados los rehenes.

Talibán ha dado su veredicto sobre el plan de negociación. "Estábamos dispuestos a mantener negociaciones directas con surcoreanos desde el principio ... Ahora nuestro consejo directivo nombró una delegación para negociar con ellos", afirmó por teléfono a la ''AFP'' Yusuf Ahmadi, un portavoz del grupo.

Aún no se ha acordado un lugar, aunque los Talibanes han rechazado una oferta de hacer las negociaciones en un centro de reuniones de la OTAN.

El gobierno afgano ha aceptado que se haga la reunión. "Una delegación de diplomáticos surcoreanos está a punto de reunirse con los talibanes para negociar directamente y estudiar los medios para la liberación de sus compatriotas", dijo Mirajuddin Pattan, el gobernador de la provincia de Gazni.

Se piensa que el grupo mantiene cautivos a los rehenes en un pueblo a 10 km del centro de la ciudad de Gazni. Se ha buscado en los hogares cercanos y se han sellado las rutas para encontrar al grupo.

El Talibán ya ha matado a dos de los rehenes que había capturado inicialmente, identificados como Bae Hyung-kyu y Shim Sung-min. El grupo insurgente exige la liberación de ocho militantes encarcelados pertenecientes al grupo por el mismo número de rehenes surcoreanos. Sin embargo, el gobierno afgano ha rechazado firmemente esta propuesta y exige la liberación de las 16 mujeres bajo los "valores del Islam".

El grupo de surcoreanos había ido a Afganistán en una misión cristiana y fue secuestrado por Talibán hace 15 días. El 25 de julio, el grupo asesinó al primer rehén y líder del grupo, Bae Hyung-kyu. El 30 de julio, el grupo ejecutó al segundo rehén, Shim Sung-min.

El grupo había dado un ultimátum tras ejecutar al segundo rehén; sin embargo, ayer el gobierno afgano pidió un aplazo de 48 horas. El plazo caducó pero el portavoz del grupo declaró que los 21 rehenes seguían con vida. Sin embargo, la tensión sigue en alza luego de que Talibán amenazara con matar "en cualquier momento" a otro rehén.

El gobierno afgano ha declarado que "por principio" el gobierno se negaba a negociar con grupos "terroristas", y también declaró que se negaba a fin de "no incitar a la toma de rehenes".

Junto con los 21 rehenes surcoreanos "aún vivos", el grupo rebelde aún tiene cautivo un ingeniero alemán junto con cuatro trabajadores afganos quienes fueron secuestrados en la Provincia de Vardak