WEIMAR (Germany) - PRESIDENT Barack Obama witnessed the Nazi ovens of the Buchenwald concentration camp on 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.

VIDEO Obama remembers Holocaust victims (2:04)

'These sites have not lost their horror with the passage of time,' Mr 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,' Mr Obama said.

He also said he saw, reflected in the horrors, Israel's capacity to empathise with the suffering of others, which he said gave him hope Israel and the Palestinians can achieving a lasting peace.

Mr 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, Mr 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.'

Fresh from visits to Saudi Arabia and Egypt, Mr 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 recognise 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.' -- AP