Amnesty International says it has gathered evidence suggesting that Iraqi government forces tortured and killed villagers near the northern city of Mosul.
The London-based human rights group said up to six people were killed execution-style in late October in several villages south of Mosul due to suspected links with the Islamic State (IS) extremist group.
Iraqi forces, backed by U.S. air strikes and Kurdish Peshmerga fighters, have been battling to retake Mosul from the militants.
"Men in federal police uniform have carried out multiple unlawful killings, apprehending and then deliberately killing in cold blood," said Lynn Maalouf, deputy director for research at Amnesty's Beirut regional office.
"In some cases the residents were tortured before they were shot dead execution-style," she added.
The rights group has called on the Iraqi government to investigate the deaths.