Pope Benedict appealed for the release of South Korean hostages held in Afghanistan on Sunday, condemning the exploitation of innocent people as a "grave violation of human dignity".

Taliban rebels abducted the Christian volunteers from a bus south of Kabul 10 days ago. They killed the leader of the group on Wednesday, and say the remaining 22 hostages will meet a similar fate unless militant prisoners are freed.

"Unfortunately the usual practice of exploiting innocent people for their own ends is spreading among armed groups," the Pope told a crowd gathered at his summer residence outside Rome.

"It is a grave violation of human dignity that clashes with every elementary norm of civility and rights and gravely offends divine law."

The Pope, who began the passage with a reference to Afghanistan, said he appealed to the "authors of such criminal acts" to stop their activities and return their victims unharmed.