President Barack Obama has condemned what he calls the "ignorant and hateful" rhetoric of those who deny the Holocaust.
Mr Obama did not mention anyone by name as he toured a former Nazi concentration camp in Germany.
However, he earlier told NBC News that President Mahmoud Ahmadinejad of Iran should visit the site.
President Ahmadinejad on Tuesday described the Holocaust as a "big deception".
Mr Obama visited the former Buchenwald camp on Friday after making a major address to the Muslim world in Cairo on Thursday.
He vowed to forge a "new beginning" between the United States and Islam, and to revive Middle East peace talks.
He placed a white rose on a site where survivors erected a temporary monument after Buchenwald's liberation by American troops in April 1945. An estimated 56,000 people were killed there.
Mr Obama was accompanied two survivors of the Holocaust and German Chancellor Angela Merkel.