Dr Harith al-Dhari, secretary general of the AMS, strongly condemned the execution, describing the slain captives as "simple people" who did not deserve their fate.
"Speaking from [a] religious and humane point of view, I was the first among those calling on the kidnappers to release the hostages," he told Aljazeera.
"We are against killing hostages, particularly if it has been a group execution. Those 12 Nepalese hostages are simple people. They might have been deceived to serve the occupation forces," he said.
"I wish they had not been treated so cruelly."
He also called for fair treatment of all peoples in Iraq, irrespective of religion and creed.
"Through Aljazeera, I call on all groups who 'claim' they are resistance and Islamic groups to be more merciful, just and humane while dealing with captives despite their religion, trends and reasons behind their presence in Iraq," he said.
Aljazeera said that a website had published video and pictures allegedly depicting the execution of 12 Nepalese captives seized by an anti-occupation group two weeks ago.
This photo of the dead Nepalese
captives was posted on a website
"We have carried out the sentence of God against 12 Nepal[ese] who came from their country to fight the Muslims and to serve the Jews and the Christians ... believing in Buddha as their god," the statement by the military committee of the Army of Ansar al-Sunna said.
The group posted a series of pictures showing two masked men, one in camouflage, holding down a hostage. The men then appeared to slit his throat and hold his severed head aloft.
Other pictures showed a group of men lying face down, with what appeared to be bullet wounds in their back.
The statement was accompanied by a video.
Cooks and cleaners
The group had earlier announced the capture of the 12 Nepalese on 19-20 August because of their cooperation with US occupation forces in Iraq.
The Nepalese workers had apparently entered Iraq to work as cooks and cleaners for a Jordanian company catering to US barracks.
"We are displaying the pictures of the Nepalese prisoners to set an example for anyone who allows himself to fight Islam and Muslims and cooperates with the occupying crusader forces in Iraq," said a statement issued along with the pictures.
Self-proclaimed anti-occupation groups have waged a campaign of kidnapping in Iraq aimed at driving out companies, individuals and troops supporting US forces and the American-appointed Iraqi interim government.