U.N. officials in Iraq stepped up pressure on the United States on Thursday to prosecute any unjustified killings of Iraqi civilians by private security contractors, saying such killings could amount to war crimes or crimes against humanity if "done in cold blood."

While Americans are unlikely to face such charges, the words served as a harsh rebuke as outrage spreads over what many Iraqis perceive as overly aggressive behavior of the heavily armed foreigners protecting U.S. government-funded work.

"For us, it's a human rights issue," said Ivana Vuco, a human rights officer with the U.N. Assistance Mission to Iraq, or UNAMI. "We will monitor the allegations of killings by security contractors and look into whether or not crimes against humanity and war crimes have been committed."

The warnings followed two high-profile cases of shooting incidents involving private contractors on the streets of Baghdad.

On Tuesday, guards working for the Australian-owned security company Unity Resources Group fired on a car as it approached their convoy, killing two women civilians. The company said its guards feared a suicide attack and fired only after issuing several warnings for the car to stop.

Also, Blackwater USA is under scrutiny for an incident in which Iraqi officials say 17 Iraqis died after Blackwater guards opened fire in central Baghdad on Sept. 16. The company has said its guards were responding to an armed attack.

"t signed the treaty backing the International Criminal Court in The Hague, Netherlands — the world's first global war crimes court.

Under international law, a war crime is a violation of the laws of war by any person or persons, military or civilian. War crimes include such acts as mistreatment of prisoners of war or civilians. A crime against humanity is a broader term that involves an act of persecution or any large scale atrocities against a body of people.

UNAMI called on the U.S. government to establish mechanisms to hold security contractors accountable for unjustified killings and to ensure that offenses committed in Iraq "by all categories of U.S. contractor employees" are subject to prosecution.

An order issued by the U.S.-led Coalition Provisional Authority in 2004 before the Iraqi government gained sovereignty gives American security companies immunity from Iraqi prosecution on issues arising from their contracts.

The U.N. said the order "enables the U.S. government to waive a contractor's immunity" but "to UNAMI'S knowledge it has not done so to date." The U.S. Embassy said it was reviewing the report and had no immediate comment.

The UNAMI report, which covered the period from April 1 to June 30, also expressed concern that too many Iraqi civilians are being killed in U.S. military operations as part of a security crackdown in Baghdad and surrounding areas.

U.S. airstrikes allegedly killed at least 88 Iraqi civilians and many more died in raids by American ground forces during a military crackdown in Baghdad and surrounding areas, according to the 36-page report.