The United Nations has refused a U.S. request to assist Iraqi judges and prosecutors seeking to try former Iraqi leader Saddam Hussein and his top lieutenants for war crimes, saying that a new Iraqi special tribunal includes a death penalty provision opposed by the United Nations and fails to meet the minimum standards of justice.

The Bush administration appealed to the U.N. war crimes tribunal for the former Yugoslavia to send some judges and prosecutors to a training conference in London for members of the Iraqi tribunal. But U.N. Secretary General Kofi Annan's office sent the court's chief prosecutor, Carla Del Ponte, a letter barring her staff from attending the week-long conference, which ended Monday, according to U.N. spokesman Stephane Dujarric.

"The United Nations noted that serious doubts exist regarding the capability of the Iraqi special tribunal to meet relevant international standards," Dujarric said at a news conference at U.N. headquarters Friday. He added that Annan maintains that "U.N. officials should not be directly involved in lending assistance to any court or tribunal that is empowered to impose the death penalty."

The United Nations was constrained in its ability to cooperate with the court without a "specific mandate" from "a competent political organ," such as the U.N. Security Council or the General Assembly, Dujarric said.

The decision was a blow to the United States and Iraq's interim government, who had hoped that a U.N. imprimatur on the court's activities would lend it greater international credibility. In a meeting at U.N. headquarters last month, interim Iraqi Prime Minister Ayad Allawi personally appealed to Annan to support Iraq's efforts to bring the country's former leaders to justice. But Annan warned Allawi that the United Nations has serious concerns about the statute that established the court, which allows the death penalty, according to a U.N. official.

The U.N. decision, which was first reported in the New York Times, irked Bush administration officials, who argued that U.N. participation in the conference could help the Iraqis develop the expertise to conduct fair trials that the United Nations claims they lack. They also noted that the United Nations has supported judicial reform efforts in countries that have the death penalty, including Rwanda and Afghanistan.

A senior U.S. official, who spoke on the condition of anonymity because of the ongoing diplomacy, said the Iraqi tribunals would proceed without U.N. support, with the first trials against Hussein's associates starting in the new year.

State Department spokesman Adam Ereli said Friday that a June 2004 Security Council resolution authorizing Annan to strengthen the rule of law in Iraq provides a legal basis for the United Nations to support the tribunal. "There is in our view a clear mandate for their involvement, not only in the political future of Iraq but also in contributing to the rule of law," Ereli said.

The Iraqi tribunal was established in December 2003 by the U.S.-appointed Iraqi Governing Council to prosecute former Iraqi officials responsible for the worst abuses under Hussein's rule. Since the Iraqi-led court's inception, international legal experts have questioned the ability of Iraq's lawyers, who have little experience in handling complex war crimes cases, to conduct a fair trial.

The court's founding statute has also alarmed U.N. lawyers and independent human rights advocates who say that it denies the accused access to an attorney during interrogations and court appearances, and that it inherits a 1971 Iraqi judicial code that permits the admission of testimony obtained through coercion.

"There are real problems in the tribunal's statute that contradict U.N. fair trial standards," said Richard Dicker, an expert on the tribunal at Human Rights Watch. Dicker said many of the conflicts now arising over the court could have been settled last year if the U.S.-led military coalition had allowed the United Nations to participate in the statute's drafting.

"They refused to do that," Dicker said. "Now they are looking to bring in the U.N. at the second-to-the-last scene in the play, when everything has already been decided."