Militias fighting alongside Iraqi troops against Islamic State are committing war crimes using weapons provided to the Iraqi military by the US, Europe, Russia and Iran, Amnesty International said on Thursday.
The rights group said that the predominantly Shia Muslim militias, known collective as the Hashid Shaabi, were using weapons from Iraqi military stockpiles to commit war crimes including enforced disappearances, torture and summary killings.
Hashid Shaabi rejected Amnesty's accusation as "lies".
Parliament voted for the Hashid to formally become part of Iraq's armed forces in November but the session was boycotted by Sunni Muslim representatives, who worry the move will entrench Shia majority rule as well as Iran's regional influence.
Iraqi and Western officials have expressed serious concern about the government's ability to bring the Shia militias under greater control.
"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," Amnesty researcher Patrick Wilcken said.
States wishing to sell arms to Iraq should ensure strict measures to ensure weapons will not be used by militias to violate human rights, he added in a statement.
Hashid spokesman Ahmed al-Assadi denied Amnesty's report.
"These lies falsify truths and contribute directly or indirectly to the continuation of struggles that the Iraqi people and the people of neighbouring countries suffer from," he told a news conference aired by state television.
"This is very clear in this report when it is purposefully slandering an official government institution," he added, calling for an inquiry into Amnesty's sources.
Amnesty cited nearly 2-1/2 years of its own field research, including interviews with dozens of former detainees, witnesses, survivors, and relatives of those killed, detained or missing.
Its report focused on four powerful militia groups, most of which receive backing from Iran: the Badr Organisation, Asaib Ahl al-Haq, Kataib Hezbollah and Saraya al-Salam.
The Hashid deny having sectarian aims or committing widespread abuses. They say they saved the nation by pushing IS back from Baghdad's borders after the army crumbled before the jihadists' lightning advance in 2014.
There have been few accusations of serious abuses by the Hashid since the start of a major offensive on October 17 to retake the northern city of Mosul from IS. Various Hashid groups have joined in that battle, and a top US general told The Daily Beast last week they had been "remarkably disciplined".