SEOUL, July 31 (UPI) -- The killing of a second hostage by Taliban militants in Afghanistan has embarrassed South Korean officials who were making desperate efforts to negotiate the safe return of the South Koreans in their 13th day of captivity Tuesday.

The Seoul government bitterly denounced the Taliban insurgents for killing anther hostage and warned that it would not "tolerate" any further "brutal" acts of harming innocent South Koreans.

But officials acknowledged that they have no effective means to secure the safe release of the remaining 21 hostages because the kidnappers are calling for the Afghan government to swap jailed Taliban insurgents for the South Korean captives, beyond Seoul's diplomatic ability.

South Korea has employed various diplomatic channels and dispatched a special presidential envoy to Afghanistan to seek a breakthrough in the hostage crisis, but its "all-out" efforts have failed to prevent the killing of another hostage.

Still worse, the Taliban kidnappers have threatened to kill more hostages if their demand of trading prisoners for hostages is not met by the Wednesday noon deadline, sparking shock and outrage among families of the hostages, mostly young Christian aid volunteers including nurses and medical students.

The kidnappers said they executed one more hostage on Monday because their demand for the release of Taliban prisoners was dismissed by the Afghan government and they threatened to start killing the rest of the South Koreans if this demand is ignored.

Seoul's Foreign Ministry identified the second victim as Shim Sung-min, a 29-year-old Christian aid volunteer. Shim, a former technology firm worker in Seoul, has done volunteer work for the poor and the disabled in his country, according to his family members.

On Sunday, Shim relayed the plight of the captives in a Taliban-arranged telephone interview with Japan public broadcaster NHK, but he was shot dead the following day. His bullet-riddled body was found on a road near the city of Ghazni, where 23 South Korean aid workers were kidnapped on July 19 on a highway as their bus traveled from Kabul to the southern city of Kandahar. They were members of a Protestant church group on a 10-day relief mission.

Shim's death comes a week after the Taliban militants shot dead Bae Hyung-kyu, who was leading the South Korean group. His body arrived in Seoul on Monday amid pleas for the release of hostages.

"The kidnappers are demanding the release of prisoners in Afghan jails in exchange for South Korean nationals. But this demand is not within the power of the Korean government because it doesn't have any effective means to influence the decisions of the Afghan government," said a government statement read by presidential spokesman Cheon Ho-seon.

The 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," stone-faced Cheon said. "The government makes it clear that it will not tolerate any further acts of harming innocent Koreans and hold the perpetrators responsible."

South Korea also called for the Afghan government to show maximum flexibility and hold serious talks over the proposed hostages-for-prisoners deal. "The position of the Afghan government is the most important to resolve the hostage situation," Cheon said.

He indirectly called for the United States to exert its influence over the Afghan government and to be more lenient with its traditional policy that refuses to negotiate with terrorists.

"The government is well aware of how the international community deals with these kinds of abduction cases. But it also believes that it would be worthwhile to use flexibility in the cause of saving the precious lives of those still in captivity and is appealing to the international community to do so," Cheon said.

The Afghan government, which was criticized for releasing five Taliban prisoners in exchange for the freedom of an Italian journalist earlier this year, has rejected the hostages-for-prisoners deal for the South Koreans.

Analysts in Seoul said the United States holds the key to resolving the hostage crisis as it has strong influence on the Kabul government led by President Hamid Karzai which replaced the Taliban regime in late 2001 on the back of U.S.-led allies.

"The Afghan government is under U.S. influence, so South Korea is urged to persuade the United States to exert its influence on Kabul," said Chang Byong-ok, a Middle East expert at Hankuk University of Foreign Studies in Seoul.

Families of the hostages, shocked by the killing of the second captive, held a news conference, calling on the United States to play a role in securing the freedom of the remaining hostages.

Civic activists and politicians gathered in front of the U.S. Embassy in Seoul to urge Washington to assume a more active role in the hostage crisis, saying South Korea has dispatched troops to Afghanistan to help U.S. campaigns in the war-torn country.