Afghanistan's Taliban was holding out Saturday for a neutral venue for talks with South Korea over the fate of 21 hostages they are threatening to kill.

The Al-Qaeda-backed militants, who are demanding that some of their men are freed from jail in exchange for the captives, have agreed to talks with the South Koreans, but are refusing to meet them in government-controlled territory.

The South Korean aid workers, most of whom are female, are said to be ill after being held for more than two weeks in sweltering southern Afghanistan.

Two are said to be in a serious condition, but the hardliners on Friday refused to allow an Afghan medical team access to them.

The dragging crisis was set to overshadow talks beginning on Sunday in the United States between Afghan President Hamid Karzai and his US counterpart George W. Bush.

South Korea is pressing the US to intervene in the crisis and has sent eight senior legislators to Washington to rally international support for its efforts to save the Christian aid workers. Two of the group have already been killed.

The rebels say they have been in regular contact with South Korea, which has told them it is doing what it can to pressure Afghanistan and the US to drop their objections to a prisoner exchange.

"They told us that they are in negotiations with the Afghan and American governments to convince them to free Taliban prisoners in exchange for the South Korean hostages," Taliban spokesman Yousuf Ahmadi told AFP Friday.

The Taliban would agree to talks if they were held in areas that the rebels control, in another country or under a UN guarantee of a "safe return" for its negotiators, he said.

The hardliners said after the latest deadline expired Wednesday they had not killed any more hostages, as they waited for direct talks with the South Korean delegation.

Seoul has however made it clear it has little room for manoeuvre.

"The Korean government is not in a position to give a direct answer to the Taliban's demand that its prisoners be swapped for Korean hostages," presidential spokesman Cheon Ho-Seon said Friday.

The Afghan government has refused to release Taliban fighters, saying it could encouraging kidnappings.

The United States criticised the government over a prisoner exchange in March that has been blamed for a recent rash of abductions, some said to have been carried out by criminals.

A 62-year-old German engineer is also being held, along with four Afghans, by separate militants who are said to have close links to the Taliban. He was seized with another German, who collapsed and was then shot dead.