Bertrand Ramcharan, the acting high commissioner for human rights, acknowledged that the removal of Saddam Hussein represented "a major contribution to human rights in Iraq" and that the United States had condemned the conduct and pledged to bring violators to justice.
"Everyone accepts the good intentions of the coalition governments as regards the behavior of their forces in Iraq," he said in a 45-page report issued at the agency's headquarters in Geneva.
But, Mr. Ramcharan declared, after the occupation of Iraq, "there have sadly been some violations of human rights committed by some coalition soldiers."
In an apparent reference to the incidents of abuse at Abu Ghraib prison and to cases where Iraqi prisoners have died in detention, Mr. Ramcharan said that "willful killing, torture and inhuman treatment" represented a grave breach of international law and "might be designated as war crimes by a competent tribunal."
He said it was a "stark reality" that there was no international oversight or accountability for the thousands of detainees, the conditions in which they were held and the manner in which they were treated.
To correct this situation, he said, the coalition authorities should immediately appoint "an international ombudsman or commissioner." That person would be charged with monitoring human rights in Iraq and producing periodic reports on "compliance by coalition forces with international norms of human rights and humanitarian law."
In its passages about the fall of Saddam Hussein, the report said that the invasion of Iraq "removed a government that preyed on the Iraqi people and committed shocking, systematic and criminal violations of human rights." It also noted approvingly that Iraqis had gained a freedom of expression never enjoyed during the years of Hussein rule.
Mr. Ramcharan, a British-educated trial lawyer from Guyana and an adjunct professor of international human rights law at Columbia University, has been a United Nations official for 30 years. He has been the acting commissioner since Sergio Vieira de Mello, the high commissioner, went to Baghdad as chief of the United Nations mission there last May on what was supposed to be a four-month assignment.
Mr. Vieira de Mello was killed in the bombing of the United Nations' Baghdad headquarters last August, and Secretary General Kofi Annan subsequently removed all international staff members from Iraq.
The human rights experts had hoped to go to Baghdad in compiling their report, but instead ended up interviewing people outside Iraq in regional capitals.
There was no immediate reaction from the United States to the report, but the White House's top lawyer warned two years ago that American officials could face prosecution for war crimes because of the unorthodox tactics of detaining Taliban and Al Qaeda suspects in Afghanistan.
The confidential Jan. 25, 2002, memo, first reported last month by Newsweek magazine, was written by the White House counsel, Alberto R. Gonzales, and urged Bush administration officials to declare captives in the war on terror exempt from the Geneva Convention. It said that otherwise, Americans might be subject to "unwarranted charges" of committing or fostering war crimes.
Critics have argued that the Bush administration's decision not to grant suspected Al Qaeda and Taliban fighters prisoner-of-war status under the Geneva Convention created the climate under which the interrogation abuses at Abu Ghraib prison occurred.
The report also comes at a moment when the United States has been hoping to obtain a Security Council resolution shielding American troops serving in United Nations-approved operations from prosecution before the International Criminal Court. The multinational force remaining in Iraq after the transfer of power to Iraq at the end of this month will be such a United Nations-sanctioned force.
Earlier this month, the United States withdrew its bid for the resolution exempting its soldiers when China indicated that it might veto the motion. In announcing the diplomatic move, China's United Nations ambassador, Wang Guangya, said he did not want to support a resolution that might grant immunity to people committing abuses like those uncovered at Abu Ghraib.