Shi'ite cleric Moqtada al-Sadr, who led an uprising against U.S.-led forces in Iraq, said on Friday militants were not justified in beheading a South Korean hostage.
A group led by Jordanian militant Abu Musab al-Zarqawi beheaded a South Korean hostage in June after Seoul refused the group's demands to withdraw its troops from Iraq.
"If you knew politics and religion, you would not have cut off his head," Sadr told Friday prayers.
"There is no religion or religious law that punishes by beheading. True, they are your enemies and occupiers, but this does not justify cutting off their heads," Sadr said in Kufa, next to southern Iraqi city of Najaf.
Sadr has been a fierce opponent of occupation, but since calling off a rebellion by his Mehdi Army militia earlier this year, he has shown signs of moderating his stance, seen as a bid to rejoin the political process.
Zarqawi's group has also beheaded an American and at least one Bulgarian. Bulgaria said it was investigating a decapitated corpse that was found on Thursday to determine if it was the body of a second Bulgarian that was being held by the group.
Sadr said he was still committed to seeing an end to the presence of foreign troops in Iraq. U.S.-led forces handed sovereignty to an interim Iraqi government on June 28, but 160,000 U.S. and other foreign troops remain in the country.
"We are suffering and will continue to suffer as long as our country is occupied. We want our country freed from occupation. We will offer our blood to sacrifice for this country, and don't imagine I will ever yield to the occupier," he said.