Iraq's Sunni Muslim officials and religious leaders accused Shiite militias of burning a Sunni man alive in an incident they say show the risk of using the gunmen to fight the militant group Islamic State.

A video circulated on social media networks purportedly shows fighters from the Imam Ali Brigades, a Shiite militia, smiling and laughing while a man is burned alive near a town in Anbar province, Iraq's biggest. It wasn't possible to verify the authenticity of the video.

Iraqi Prime Minister Haidar al-Abadi, a Shiite, ordered the militias to deploy in Anbar after Islamic State militants overran government and tribal forces in May to seize Ramadi, the provincial capital, located about 110 km (68 miles) west of Baghdad.

Sunni leaders and international human rights groups have accused the militias, some of which are backed by Iran, of sectarian violence in other areas of the country.

"Such actions will increase fears among Sunnis and raise so many question marks on where this country is heading," said Hamed al-Mutlaq, a Sunni lawmaker who said he had watched the video.

The Association of Muslim Scholars in Iraq, a religious body representing Sunnis, described the burning as a crime against humanity.

Saad al-Hadithi, an Iraqi government spokesman, couldn't be immediately reached for comment.