(Corrects heart attack reference to "stress and strain")

By Sayed Salahuddin

KABUL, July 21 (Reuters) - Afghan Taliban rebels said they had killed two German hostages on Saturday, but Germany's foreign minister said one of the hostages was still alive and the other had died from "stress and strain".

A Taliban spokesman, Qari Mohammad Yousuf, also said the militants would start killing the 23 Korean hostages they held if South Korea did not withdraw its troops from Afghanistan and the Afghan government did not release Taliban prisoners.

The spokesman said the two Germans had been killed after similar demands over Taliban prisoners and for Germany to withdraw its troops had not been met.

German Foreign Minister Frank-Walter Steinmeier told reporters in Berlin there had been a lot conflicting information about the fate of the hostages, but analysis suggested one hostage was alive.

"We have to assume that one of the two hostages died while being held hostage and all indications are that he was not murdered, but that he died of stress and strain ... we will do everything possible to save the life of the second hostage."

He said German officials were working with the Afghan authorities to secure the release of the remaining hostage.

Germany's Bild am Sonntag newspaper quoted unnamed German government sources as saying Yousuf did not speak for the hostage takers. German intelligence sources told Bild the spokesman had nothing to do with the kidnappers, it said.

"He may be someone trying to take advantage of the situation," Bild said in a preview of an article for Sunday.

Taliban spokesman Yousuf insisted the two Germans were dead.

"This is just words, they are trying to console themselves," he told Reuters by telephone from an undisclosed location. "We have also killed five of the Afghan hostages we kidnapped with the Germans."

The Taliban would start killing the 23 Korean Christians, he said, setting a deadline of 1430 GMT on Sunday.

"If our conditions are not met, then they will have the same destiny as the Germans and Afghans," Yousuf said.

He said the Taliban had yet to decide what to do with the bodies of the two Germans. Seized in Wardak province southwest of the capital, Kabul, he said the pair were shot dead in Ghazni province just to the east.

A day after kidnapping the Germans, the Taliban seized the group of South Korean Christians travelling in a bus in Ghazni.

Ghazni and Wardak provinces have seen a marked escalation of violence in the last month as Taliban militants have moved in from the south. Residents say government troops only hold the major towns and much of the countryside is beyond their control.

SOUTH KOREAN TEAM

U.N. Secretary-General Ban Ki-moon expressed concern to Afghan President Hamid Karzai earlier on Saturday.

"In particular, Mr Ban asked for President Karzai to be personally engaged so as to achieve a prompt resolution," a U.N. statement said.

A South Korean government team plans to arrive in Afghanistan on Sunday and meet Karzai and the Taliban to try to find an "understanding" to free the Korean Christians, a Korean embassy official said on Saturday.

Taliban spokesman Yousuf welcomed the team's visit and said South Korea has to do its best to save its nationals.

South Korean President Roh Moo-hyun has called for the release of the hostages, saying they were medical volunteers.

South Korea has no combat troops in Afghanistan, but has a contingent of 200 engineers, doctors and medical staff, mostly serving with foreign troops. Roh said they would stay in Afghanistan until their mission was complete.

The kidnapping of the Koreans is the biggest group of foreigners seized so far in the militant campaign to oust the U.S.-backed government and force out foreign troops.

German Chancellor Angela Merkel in a newspaper interview published on Saturday said there was no question of withdrawing troops from Afghanistan.

At the start of July, a German was taken hostage and then freed after a week. In October 2006, two German journalists were killed while camping in the relatively stable northern part of the country where there are more than 3,000 German troops stationed as part of a NATO peacekeeping force.

If confirmed, the death of the two German hostages will likely prompt German opposition parties to increase their calls for Germany to pull its troops out of Afghanistan. (Additional reporting by Soyoung Kim in Seoul and Louis Charbonneau in Berlin)

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