United Nations ? The U.N.’s top human rights official said Friday that U.S.-led forces had committed serious human rights violations in Iraq since the occupation, and incidents at Abu Ghraib prison could be considered war crimes.

While crediting the United States for ending the “shocking and systematic” human rights abuses that occurred under Saddam Hussein, Acting High Commissioner for Human Rights Bertrand Ramcharan, said that the occupation forces had mistreated many ordinary Iraqis, and called for an ombudsman to monitor their behavior.

In the 45-page report released in Geneva, Ramcharan noted the complexity of Iraq’s situation, marked by a murky political transition, acts of terrorism and civilians getting caught up troops’ efforts to contain insurgents’ attacks.

But he said there had been “serious violations of human rights” under the Coalition Provisional Authority, with many Iraqis detained “without anyone knowing how many, for what reasons, and how they were being treated.”

The report cited interviews conducted by U.N. employees and foreign aid groups with Iraqis who spoke of arbitrary arrests, detention and beatings as an ongoing phenomenon since the U.S.-led invasion of March 2003.

Sergio Vieira de Mello, the former human rights chief who was acting as U.N. special envoy to Iraq last year, had reported allegations of torture and ill treatment of prisoners to the American administrator of Iraq, L. Paul Bremer, last summer. Ramcharan became the acting human rights chief when Vieira de Mello died in August in the bombing of the U.N.’s Baghdad headquarters.

Ramcharan, a longtime U.N. official, commissioned an investigation in April, because he was concerned that Iraq has been unmonitored since Saddam’s regime fell in March 2003. Ramcharan urged that a monitoring system be set up immediately in Iraq, that would allow for regular investigations of all detention facilities in the country.

Referring to incidents of prisoner abuse by U.S. soldiers and contractors at Abu Ghraib, Ramcharan said “willful killing, torture or ill treatment” of detainees was a grave breach of international law. Such acts, he added, “might be designated as war crimes by a competent tribunal.”

One U.S. soldier has pleaded guilty in connection with the incidents and sentenced to a year in jail. But American personnel are unlikely to be tried in an international tribunal because the United States has not signed on to the International Criminal Court.

The U.N. report’s finding are consistent with recent investigations by the Red Cross, Human Rights Watch and Amnesty International.

U.S. and British officials were allowed to comment on the report before it was released, but Ramcharan’s spokesman, Jose Diaz, said they did not pressure the agency to make changes.

The report emphasized that the abuses by U.S. forces were on a different scale from the long-standing, systematic violations conducted under the former Iraqi regime, and recognized that removal of Saddam’s government “must be counted as a major contribution to human rights in Iraq.”