The hardline Islamic militia wants the government to free at least eight Taliban prisoners in Afghan jails, a demand government negotiators have rejected.

"If our demands are not met by then, we will start killing the rest of the South Koreans," Taliban spokesman Yousuf Ahmadi told AFP.

The bloodied corpse of the second hostage to be killed since 23 were kidnapped nearly two weeks ago was found early Wednesday in the southern province of Ghazni, about 140km south of Kabul.

The body was dumped in a field just off a main road, with the hands tied and bullet wounds to the head.

South Korea's foreign ministry identified the victim as Shim Sung-Min, 29. The country reacted with outrage.

"The Korean government strongly condemns and urges an immediate end to these heinous acts of killing innocent people in order to press for demands that it can't meet," the presidential office said.

Media reports said Shim had quit his job at a Seoul IT company two months ago to become a teacher to the disabled at a Seoul church, which had then sent him to Afghanistan on an aid mission.

The body of pastor Bae Hyung-Kyu, 42, who had been leading the group on the mission to the risky south, was found in the same area last Wednesday.

The South Korean government urged the international community to "use flexibility" to save the remaining 21 hostages.

But a spokesman for President Hamid Karzai said the Taliban's demands should "as a principle" not be accepted as this would only encourage kidnapping.

If the government continued "responding positively to their request and to the demands of the terrorists, we'll face more problems," Humayun Hamidzada said.

"I think as a principle we shouldn't encourage kidnapping by accepting their demand," he said, adding: "This shouldn't become an industry."

Five Taliban prisoners were freed from jail in March in a widely criticised deal that saved an Italian journalist kidnapped by the Taliban. The rebels nevertheless killed two Afghans captured with him.

Analysts warned at the time that the exchange could encourage kidnapping by militants and criminals alike. Karzai vowed then that such swaps would not be repeated.

The Taliban has said it is also holding a German engineer who was kidnapped in Wardak province near Kabul a day before the South Koreans. It has also demanded the release of prisoners to save his life.

Police said Tuesday that a four-man Afghan health team missing near the southern city of Kandahar since Monday had been kidnapped but it was not clear who might be holding them.

Government negotiators said meanwhile they were continuing efforts to free the remaining South Koreans, 16 of whom are women, but there had been little movement.

"Taliban are not responding very nicely," Mahmood Gailani told AFP. "We are hopeful to find a solution, but it's very difficult."

The government side was still asking the militants to release the female captives while the rebels stood firm on their demand that Taliban prisoners should be freed first, he said.

"We are asking for more time," Gailani said.

The Taliban meanwhile claimed responsibility for a suicide bombing near international troops in the capital Kabul Tuesday. The blast killed an Afghan truck driver and wounded three US soldiers, local police said.