President Barack Obama witnessed the Nazi ovens of the Buchenwald concentration camp Friday, its clock tower frozen at the time of liberation, and said the leaders of today must not rest against the spread of evil. The president called the camp where an estimated 56,000 people died the "ultimate rebuke" to Holocaust deniers and skeptics. And he bluntly challenged one of them, Iranian President Ahmadinejad, to visit Buchenwald. "These sites have not lost their horror with the passage of time," Obama said after seeing crematory ovens, barbed-wire fences, guard towers and the clock set at 3:15, marking the camp's liberation in the afternoon of April 11, 1945. "More than half a century later, our grief and our outrage over what happened have not diminished." Buchenwald "teaches us that we must be ever-vigilant about the spread of evil in our own time, that we must reject the false comfort that others' suffering is not our problem, and commit ourselves to resisting those who would subjugate others to serve their own interests," Obama said. He also said he saw, reflected in the horrors, Israel's capacity to empathize with the suffering of others, which he said gave him hope Israel and the Palestinians can achieving a lasting peace. Obama became the first US president to visit the Buchenwald concentration camp. It was, in part, a personal visit: His great-uncle helped liberate a nearby satellite camp, Ohrdruf, in early April 1945 just days before other US Army units overran Buchenwald. Earlier in Dresden alongside German Chancellor Angela Merkel, Obama pressed for progress toward Mideast peace. The US "can't force peace upon the parties," he said, but America has "at least created the space, the atmosphere, in which talks can restart." The president also announced he was dispatching special envoy George J. Mitchell back to the region next week to follow up on Obama's speech in Cairo a day earlier in which he called for both Israelis and Palestinians to make concessions in the standoff. Fresh from visits to Saudi Arabia and Egypt, Obama said that while regional and worldwide powers must help achieve peace, responsibility ultimately falls to Israelis and Palestinians to reach an accord. He said Israel must live up to commitments it made under the so-called "Road Map" peace outline to stop constructing settlements, adding: "I recognize the very difficult politics in Israel of getting that done." He also said the Palestinians must control violence-inciting acts and statements, saying that Palestinian President Mahmoud Abbas "has made progress on this issue, but not enough." Merkel, for her part, promised to cooperate on the long-sought goal. She said the two leaders discussed a time frame for a peace process but did not elaborate. "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. Elie Wiesel, a 1986 Nobel Peace Prize winner, author and Holocaust survivor whose father died of starvation at Buchenwald three months before liberation, and Bertrand Herz, also a Buchenwald survivor; accompanied Obama and Merkel at the camp. Each laid a long-stemmed white rose at a memorial. They were later joined by Volkhard Knigge, head of the Buchenwald memorial. "To this day, there are those who insist the Holocaust never happened," Obama said. "This place is the ultimate rebuke to such thoughts, a reminder of our duty to confront those who would tell lies about our history." It was a pointed message to Iran's Ahmadinejad, who has expressed doubts that 6 million Jews died at the hands of the Nazis. "He should make his own visit" to Buchenwald, Obama told NBC earlier Friday. He added: "I have no patience for people who would deny history." Separately, the president told reporters: "The international community has an obligation, even when it's inconvenient, to act when genocide is occurring." After the tour, Obama was flying to Landstuhl medical hospital for private visits with US troops recovering from wounds sustained in Iraq and Afghanistan. And he was ending the day in Paris - reuniting with his wife, Michelle, and daughters Malia and Sasha, who planned a brief holiday in the City of Light after commemorating the 65th anniversary of the Allies' D-Day invasion in France.