In an interview in Germany with NBC News shortly before making his own trip to Buchenwald to pay homage to victims of World War Two and the Holocaust, Obama was asked what he thought the Iranian leader could learn from his visit to the site.

"He should make his own visit," Obama said. "I have no patience for people who would deny history. And the history of the Holocaust is not something speculative."

This week Ahmadinejad called the Holocaust a "great deception" during a speech containing his latest verbal assault on Israel, which the Islamic Republic does not recognize.

Obama noted that his great uncle helped liberate a camp of Buchenwald during World War Two. The concentration camp in eastern Germany was created by the Nazis and an estimated 56,000 people, mostly Jews, were killed there.

President Barack Obama says the Buchenwald concentration camp "is the ultimate rebuke" to those who deny the Holocaust.

Obama visited the Nazi-era camp where 56,000 people died. He toured the memorial Friday with German Chancellor Angela Merkel and survivor Elie Wiesel. They laid roses at a memorial.

Obama said people today have a duty to confront those who deny such concentration camps existed.

A somber Obama told reporters that his great-uncle helped liberate a nearby satellite camp, Ohrdruf, just days before other U.S. Army units overran Buchenwald. Obama says his great-uncle returned from war and was unable to speak of the horrible scene.

Prodding the international community, President Barack Obama called "for all of us to redouble our efforts" toward separate Israeli and Palestinian states. "The moment is now for us to act," he declared.

Alongside German Chancellor Angela Merkel following his Mideast trip, Obama said: "The United States can't force peace upon the parties." But he said America has "at least created the space, the atmosphere, in which talks can restart."

The president announced he was dispatching special envoy George J. Mitchell back to the region next week to follow up on his speech in Cairo a day earlier in which he called for both Israelis and Palestinians to give ground in the standoff.

Obama says Israel must live up to commitments it made under the so-called "Road Map" peace outline to stop constructing settlements and the Palestinians must control violence-inciting acts and statements.

Fresh from visits to Saudi Arabia and Egypt, Obama said that while regional powers and the entire international community must help achieve peace, responsibility ultimately falls to Israelis and Palestinians to reach an accord.

Merkel, for her part, promised to cooperate on this long-sought goal. She said the two leaders discussed a time frame for a peace process but did not elaborate.

"I think that, with the new American government and the president, there is a truly unique opportunity to revive this peace process or, let us put this very cautiously, this process of negotiations," Merkel said.

Added Obama: "I think the moment is now for us to act on what we all know to be the truth, which is each side is going to have to make some difficult compromises."