South Korean authorities are scrambling to confirm Taleban claims the insurgent group has killed one of 23 hostages in Afghanistan.
Meanwhile, unconfirmed South Korean media reports say some of the hostages may have been freed.
South Korean officials say it may take time to confirm claims that Taleban insurgents have killed one of their hostages.
Meanwhile, South Korean media quote a government official saying at least eight other hostages have been released.
The self-described Taleban spokesman Qari Yousuf Ahmadi informed reporters of the killing Wednesday. Earlier in the day, Ahmadi warned in a telephone call the hostages were in grave danger.
Ahmadi says the Taleban is not satisfied with the Afghan and South Korean governments' response to demands that several Taleban prisoners be released. He declares the hostage negotiations a failure, and says "a few" of the South Korean hostages may be executed.
No formal affiliation has ever been confirmed between Ahmadi and the Taleban. Occasionally, some of his statements have proven to be inaccurate.
Regardless, South Korean officials confirm they are devoting serious attention to the threat, and to broader efforts to free the hostages.
Here in South Korea, relatives of the hostages agonized over Wednesday's threat.
Kim Hyung-seok, president of the aid foundation that organized the trip, says some family members of the 23 hostages fainted when they heard the new threat.
The Taleban kidnapped the 23 South Koreans-- 18 of them women-last Thursday.
All of them are members of a Christian church outside of Seoul. Although the church describes their activities as humanitarian, many critics describe the travelers as missionaries, who put themselves unnecessarily at risk in one of the world's hotbeds of Islamic militancy.
South Korea bans most travel to Afghanistan, where a few hundred its soldiers and civilian personnel provide non-combat support to international security efforts.
The 23 hostages made their trip illegally. Since their capture, Seoul has announced harsher penalties for travel to banned areas.
South Korea is second only to the United States in the number of Christian missionaries it sends abroad, with an estimated 12,000 engaged in projects in about 160 countries.