A protester holds a combo picture of South Korean hostages being held in Afghanistan during a rally

South Korean officials and the Taliban struggled Friday to agree on a venue for talks to save the lives of 21 hostages, as the rebels refused to allow Afghan doctors access to the group.

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

Taliban spokesman Yousuf Ahmadi told AFP there had been new telephone contact Friday with South Korean ambassador Kang Sung-Zu.

"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," Ahmadi said.

The release of Taliban prisoners has been the key demand of the extremists, who kidnapped the group of Christian aid workers, most of whom are women, on July 19 as they travelled in the country's insurgency-hit south.

Seoul has however made it clear it cannot guarantee anything.

"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.

South Korea's Yonhap news agency meanwhile cited "informed sources" saying direct talks between South Korean officials and the Taliban were likely to be held later Friday.

But Ahmadi said there had been no agreement on a venue with the Taliban rejecting a meeting in government-controlled territory.

They would however immediately agree to talks in its areas, in another country or under a UN guarantee of a "safe return" for its negotiators, he said.

The hardliners meanwhile refused to allow an Afghan medical team access to the hostages, most of whom are said to be ill -- two of them seriously.

"Of course we cannot trust them. But we can offer a very simple solution: the government can release two of our prisoners in exchange for the two Koreans who are very sick and we can see for the rest later," Ahmadi said.

The Afghan government has refused to release Taliban fighters for fear of encouraging kidnapping, and after severe criticism from the United States over a similar deal in March that has been blamed for a recent rash of abductions.

Some of the kidnaps have been carried out by criminals.

Seoul has been pressing the United States, a close ally, to help end the ordeal of the hostages, sending eight senior South Korean legislators to Washington Thursday.

Richard Boucher, the US Assistant Secretary of State for South and Central Asian Affairs, told reporters Thursday that "potential military pressures" against the Taliban were among the "many tools" available.

But Seoul has also repeatedly stated its stiff opposition to any attempt to free the hostages by military action. Officials in Kabul have denied reports of a possible military operation.

In yet another international call for the group to be freed, a major Muslim organisation in Indonesia, Din Syamsuddin, on Friday labelled the kidnapping "absolutely unjustifiable" and "in violation of Islamic principles and teaching."

Two dozen Christian pastors and clergymen prayed outside the US embassy in Seoul, urging Washington to accept the Taliban's demands to secure their release.

"The US, which massacred civilians in the name of a war against terrorism, must assume responsibility for the current situation," a statement signed by 96 clergymen said.

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.

The abductions highlight growing insecurity in Afghanistan, one of the key battlegrounds in the US-led "war on terror," nearly six years after the United States led the invasion that toppled the Taliban government for sheltering Al-Qaeda.