Some of the 22 South Koreans being held captive by the Taliban are in bad health, a purported insurgent spokesman said, as a top Korean envoy arrived for high-level talks over the hostage crisis.

The Taliban spokesman on Friday reiterated the group's demand for the release of insurgent prisoners in exchange for the captives, but said the militia had not set a new deadline in the ongoing negotiations.

Several earlier deadlines given since the South Koreans were seized on July 19 have passed without incident. One of the original 23 captives was shot to death on Wednesday, though the reasons why are not clear.

Afghan officials were optimistic the hostages could be freed without further bloodshed, although the Taliban said the captives would be killed if their demands were not met.

A South Korean presidential envoy, Baek Jong-chun, arrived for talks with Afghan President Hamid Karzai, an official from the South Korean Embassy in Kabul said. He spoke on condition of anonymity because of embassy policy.

Negotiators were struggling with conflicting demands made by the kidnappers, including the release of Taliban prisoners and ransom money.

Purported Taliban spokesman Qari Yousef Ahmadi said the militants hoped the South Korean envoy would be able to "persuade the Afghan government" to swap imprisoned militants for the captives.

"If they don't release the Taliban prisoners, then the Taliban does not have any option other than to kill the Korean hostages," he said, reiterating an earlier threat.

Some of the South Koreans were "not in good condition," Ahmadi said. "I don't know if the weather is not good for them or our food. The women hostages are crying. The men and women are worried about their future."

In Seoul, a South Korean Foreign Ministry official, speaking on condition of anonymity because of the sensitivity of the situation, said the captives were still believed to be safe and officials were trying to get medicine and other items delivered to them.

Local tribal elders and clerics from Qarabagh district where the Koreans were taken have been conducting negotiations by telephone with the captors for several days.

The South Koreans, including 18 women, were kidnapped while traveling by bus on the Kabul-Kandahar highway, Afghanistan's main thoroughfare. Ahmadi said the hostages were being held in small groups in different locations and were being fed bread, yogurt and rice.