The US's largest Muslim civil rights organization on Wednesday condemned President Donald Trump's decision to grant clemency to four former Blackwater contractors convicted of killing Iraqi civilians.

National Executive Director of the Council on American-Islamic Relations (CAIR) Nihad Awad said the decision lacks respect for the US legal system and "the sanctity of human life, especially the lives of Muslims and people of color."

"These Blackwater mercenaries were convicted of perpetrating one of the most infamous war crimes of the American occupation of Iraq. Pardoning them is an unconscionable act of moral insanity," said Awad.

The White House said late Tuesday that Trump granted clemency to 20 people, including former congressmen and the security contractors who worked for US paramilitary company Blackwater. The company has since changed its name to Academi.

Nicholas Slatten, Paul Slough, Evan Liberty and Dustin Heard, who were working in Iraq in 2007, were convicted of killing 14 Iraqi civilians in a crime that was condemned at international levels.

In March 2003, the US invaded Iraq and toppled the regime of Saddam Hussein.

The human cost of the US occupation was high with more than 100,000 civilian deaths along with evidence of torture and abuse in US-run prisons./aa