Pro-government Iraqi paramilitaries accused of war crimes are using arms from at least 16 countries, including the US and Iran, according to an Amnesty International report released on Thursday.

The predominantly Shia militias were formed in 2014 to support the Iraqi government in its fight against the Islamic State group and have since committed war crimes, Amnesty said.

The Sunni Arab community has been targeted by paramilitaries, which have acted with total impunity in carrying out extrajudicial executions, torture and enforced disappearances, among other crimes.

"International arms suppliers, including the USA, European countries, Russia and Iran, must wake up to the fact that all arms transfers to Iraq carry a real risk of ending up in the hands of militia groups with long histories of human rights violations," said Patrick Wilcken, an arms control researcher at Amnesty.

The London-based rights group used field research and analysis of photo and video evidence since June 2014 to document the arms used by the paramilitaries, which Amnesty said were manufactured in at least 16 countries.

Tanks, machine guns and sniper rifles were among more than 100 types of arms used by the groups according to Amnesty.

Weaponry has been supplied by Iraqi state institutions or with the authorities' approval, while militia members have also purchased weapons on the private market including online sales.

Iran was named as a major military sponsor of militias which are accused of serious human rights violations.

Despite the paramilitaries formally becoming part of the Iraqi military last year, Amnesty said its request to the defence ministry for details of accountability mechanisms went unanswered.

"Instead of unequivocally hailing militias as heroes fighting to put an end to IS (Islamic State) atrocities, thereby emboldening them, the Iraqi authorities must stop turning a blind eye to systematic abuses that have fed sectarian tensions," said Wilcken.

Amnesty also called on countries selling arms to Iraq to put measures in place to ensure the weapons are not used by militias guilty of abuses.