Stephen Rapp's brief remarks marked the first time that a U.S. diplomat has addressed the 110-nation Assembly of State Parties, which oversees the court's work and budget.

He also held a string of bilateral meetings and told delegates he was there to listen and learn.

Rapp underscored Washington's history of helping prosecute those responsible for atrocities dating back at least to the Nazi war-crimes trials in Nuremberg.

The world's first international war-crimes tribunal began work in 2002. It is a court of last resort to prosecute people suspected of committing war crimes in its member states, if those countries cannot or will not conduct the trials themselves.

The U.N. Security Council also can ask the court to investigate a case.

The United States refused to ratify the court's founding treaty, the 1998 Rome Statute, partly because of fears the court could become a forum for politically motivated prosecutions of troops in unpopular wars like Iraq.

But the United States has "not been silent in the face of crimes against the basic code of humanity," Rapp said. "Far from it: We have worked shoulder to shoulder with other states to support accountability and end impunity for hauntingly brutal crimes in the former Yugoslavia, Rwanda, and elsewhere."

The United States allowed the Security Council to call on the court to intervene in the Sudanese region of Darfur, clearing the way for it to issue an arrest warrant in March for Sudanese President Omar al-Bashir.