The South Korean envoy was due to arrive in Kabul to seek an urgent hearing with Afghanistan's President Hamid Karzai and US-led forces following a desperate appeal by one of the captive Christian aid workers for help.

Taliban militants had set a midday yesterday deadline to arrange the release of the remaining 22 Christian aid workers now in their ninth day of captivity. The leader of the group has already been killed.

The Islamic guerrillas are insisting on the release of eight Taliban prisoners held in Afghanistan in return for the aid workers' freedom, although Seoul has said the rebels' demands are "considerably fluid and not unified."

"We call on the government to give more value to the negotiations and speed up the process of releasing our suggested prisoners," Taliban spokesman Yousuf Ahmadi told AFP from an unknown location after the limit expired.

"They requested more time. They say they have a new Korean envoy involved in the negotiation process. But if we feel they are not honest in releasing our prisoners we will kill all of the hostages," he said.

There was no official confirmation that the envoy had arrived.