CAIRO, Egypt -- A renowned jihadi ideologue on Saturday urged the Islamic State group to release British aid worker Alan Henning, saying Islam forbids harming non-Muslims who work with relief agencies.

Abu Mohammed al-Maqdisi, who was released by Jordan in June after serving a five-year sentence on terror charges, said in a statement posted on his website that non-Muslims who aid needy Muslims should be protected.

The Islamic State group has released grisly videos showing the beheading of two American journalists and a British aid worker. It has threatened to kill Henning -- a British former taxi driver who was taken captive in December shortly after crossing into Syria from Turkey in an aid convoy -- in retaliation for U.S. and European military action against it.

Also known as Essam al-Barqawi, al-Maqdisi was the mentor of Abu Musab al-Zarqawi, the leader of al Qaeda in Iraq, who was killed in a U.S. airstrike in 2006.

Al-Maqdisi said Henning worked with a charitable organization led by Muslims which sent several aid convoys to help the Syrian people. "Is it reasonable that his reward is being kidnapped and slaughtered? ... He should be rewarded with thanks."

"We call on the (Islamic) State to release this man (Henning) and other aid group employees who enter the land of Muslims with a guarantee of protection... according to the judgment of Shariah law," he said, adding that he hoped to "protect the image of Islamic Shariah law from being disfigured."

Al-Maqdisi said Abu Qatada, a jailed Jordanian preacher described as a onetime lieutenant to Osama bin Laden, had asked the Islamic State group eight months ago to release Henning. He said Abu Qatada's son told him that the group denied holding Henning.

Al-Maqdisi went on to criticize the group for attacking fellow Muslims in Iraq and Syria, where it has carved out a self-styled caliphate by seizing territory from Syrian and Iraqi forces as well as Syrian rebels fighting to oust President Bashar Assad.

The Islamic State group has its roots in al Qaeda's Iraqi affiliate but was expelled from the global terror network over its brutal tactics and refusal to obey orders to confine its activities to Iraq.

The extremist group has been widely denounced by mainstream Muslim authorities.