The American war crimes ambassador said Thursday the U.S. is committed to ending impunity for crimes against humanity, in a speech signaling a softening of hostility toward the International Criminal Court.

Stephen Rapp's brief remarks marked the first time 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.