Iraq's U.N. ambassador Friday accused U.S. Marines of killing his 21-year-old cousin "in cold blood" during a June 25 raid in a village in the Sunni Muslim-dominated province of Anbar.

Samir S.M. Sumaidaie called on the United States to investigate the death of Mohammed Sumaidaie in "a credible and fair way to ensure that justice is done." He said the killing represents a "betrayal" of Iraqi and U.S. efforts to rebuild Iraq on a foundation of "freedom, democracy and respect for human rights and the rule of law."

The allegation comes as the United States is trying to persuade Iraq's Sunni Muslim community, which provides the largest base of support for the insurgency, to break with Islamic extremist elements in Iraq and participate in the country's political transition. The U.S. military has held a series of private meetings this summer with insurgent leaders to convince them to back the political process.

A spokesman for the U.S.-led multinational force in Camp Fallujah, in the heart of Iraq's Sunni region, issued a statement saying an inquiry had been launched into the incident, noting that the "allegations roughly correspond to an incident involving coalition forces" in the area on June 25.

"We take these allegations seriously and will thoroughly investigate this incident to determine what happened," said Maj. Gen. Stephen T. Johnson, commanding general of Multi-National Force-West. The investigation could take several weeks, according to the statement.

According to a witness account compiled by Sumaidaie, U.S. Marines and Iraqi Army units were conducting a joint series of raids June 25 in the town of Haditha. At 10 a.m., 10 Marines searching for weapons knocked on the door of Mohammed's father's house.

Mohammed, described as a "relatively shy" engineering student, "greeted them pleasantly" and led them to his father's bedroom, where the family stored an old rifle filled with blanks, according to the account.

His relatives say they do not know exactly what happened next, but they say that several family members, including Mohammed's mother, were gathered in the hallway and heard a thud in the bedroom. Meanwhile, according to Sumaidaie's account, one Marine dragged Mohammed's younger brother, Ali, through the house by his hair and beat him.

The family was asked to wait on the porch while the Marines continued searching the house for another hour, the account said. When they finished, an interpreter for the Marines asked Mohammed's mother in Arabic whether it was her son in the bedroom. The account said he told her, "They killed him!"

"In the bedroom Mohammed was found dead and laying in a clotted pool of his blood," according to the account.

Sumaidaie wrote in a statement released Friday: "If that account is broadly accurate, and I have absolutely no doubt that it is because I made sure of it, a serious crime has been committed. All indications point to a killing of an unarmed civilian -- a cold-blooded murder."

The United States' acting U.N. ambassador, Anne W. Patterson, expressed "heartfelt condolences" to Sumaidaie and asked the U.S. embassy in Iraq and senior Pentagon officials "to look into the matter immediately," said Richard Grenell, spokesman for the U.S. mission to the United Nations.

"The situation is obviously just terrible," Grenell said.