Some in South Korea want the US to intervene in the crisis

The Taleban and South Korean officials are expected to hold face-to-face talks to discuss the fate of 21 Koreans, held by the militant group for two weeks.

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.