A US-trained Iraqi army division allegedly executed several dozen men in the final throes of the battle against Islamic State militants in Mosul's Old City, Human Rights Watch said on Thursday.
The watchdog urged the US government to suspend all support for the 16th Division of the Iraqi army pending an investigation into what it called war crimes, evidence of which was seen by two international observers.
Reuters could not independently verify the claims because Iraqi authorities have restricted media access to the Old City since Prime Minister Haider al-Abadi declared victory over Islamic State on July 10. Spokesmen for the Iraqi government and military could not be reached for comment.
Islamic State made its last stand in the Old City after nine months of urban warfare with Iraqi forces who are backed by a US-led coalition.
Fighting continued there for several days after victory was declared in mid-July and videos emerged of Iraqi forces beating unarmed men and pushing one off a precipice to his death.
"Given the widespread abuses by Iraqi forces and the governments abysmal record on accountability, the US should take a hard look at its involvement with Iraqi forces," said Sarah Leah Whitson, Middle East director at Human Rights Watch.
Iraq has promised to investigate previous accusations of abuses and hold perpetrators to account.
The observers cited by HRW said they had seen a group of Iraqi soldiers who identified themselves as members of the 16th Division lead four naked men down an alleyway, after which they heard multiple gunshots.
They were told by other soldiers that the four men were Islamic State fighters.
As they were leaving the area, one of the observers saw the bodies of a number of naked men lying in a doorway, one of whom appeared to have been handcuffed and had a rope tied around his legs.
"The US military should find out why a force that it trained and supported is committing ghastly war crimes," Whitson said. "US taxpayer dollars should be helping to curtail abuses, not enable them."