South Korea on Thursday condemned the killing of one of its citizens held hostage in Afghanistan and said it will spare no blame for their abductors, a senior presidential aide said.

Taliban insurgents on Wednesday shot dead one of the 23 South Korean volunteers they took hostage last week and threatened to kill the others unless rebels are released from prison.

"The government and the people of South Korea condemn the kidnapping of innocent civilians and the atrocity of harming a human life," said Baek Jong-chun, presidential Blue House chief national security adviser.

"Harming innocent civilians can never be justified and we will never forgive this kind of inhumane act," he said in a nationally televised statement.

South Korea will soon dispatch a special envoy to Afghanistan to step up coordination with Kabul to secure the release of the hostages, he added.

The 23 South Koreans -- 18 women and five men on a volunteer project organised by a Seoul church -- were abducted on the main road south from Kabul on Friday.

On Wednesday the body of the South Korean, whose identity has not been confirmed, was found with bullet wounds.

The Taliban has accused the Afghan government and South Korean negotiators of failing to act in good faith after Kabul rejected a demand for eight named rebels to be freed from prison.

Taliban spokesman Qari Mohammad Yousuf rejected South Korean media reports that the Taliban planned to free eight of the captives.

Afghan President Hamid Karzai has pledged not to swap prisoners for hostages after being criticised at home and abroad for releasing five Taliban from jail in March in exchange for an Italian reporter.