A claim by Taliban rebels that they have killed two German hostages, has been refuted by an Afghan Foreign ministry official.

Citing Afghan security sources, Sultan Ahmad Baheen said that one of the Germans was still alive and the other had died of a heart attack, contradicting the statement from the Taliban.

A spokesman for the militant group, Qari Mohammad Yousuf, said that they had killed the Germans because their demands had not been met and that the militants would start killing the 23 Korean hostages they had also captured.

The group said that if South Korea did not withdraw its troops from Afghanistan and the Afghan government did not release Taliban prisoners they would begin killing the group of Christian volunteers, 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.

The German Foreign Ministry said it had received no independent confirmation that the hostages in Afghanistan had been killed by the Taliban.

"We are taking these statements very seriously," German Foreign Ministry spokesman Martin Jaeger said in a statement. "So far we have no independent confirmation that a hostage was murdered in Afghanistan."