(Updates with full details of speech)
JERUSALEM, May 15 (Reuters) - U.S. President George W. Bush on Thursday decried his critics' calls for negotiations with Iranian President Mahmoud Ahmadinejad as comparable to the appeasement of Adolf Hitler before World War Two.
Bush's comment in a speech to Israel's parliament appeared to be a swipe at Democratic presidential frontrunner Barack Obama, who has advocated meeting leaders of traditional U.S. foes such as Iran and Cuba without preconditions.
It also followed a visit to the Middle East by former President Jimmy Carter in which he met Hamas leaders, who are shunned by Israel and Washingtion, and urged efforts to draw the militant group into the Israeli-Palestinian peace process.
Bush used his speech to ratchet up his rhetoric against Iran, saying Washington stood by Israel in opposing what he called Tehran's "nuclear weapons ambitions".
Bush's words also had implications for the U.S. presidential campaign, which has increasingly overshadowed him in his final year in office as the Democratic candidates have sharpened their criticism of his foreign policy.
Bush, who has refused any contact with Ahmadinejad, said the Iranian president "dreams of returning the Middle East to the Middle Ages and calls for Israel to be wiped off the map", and lumped him together in an anti-Israel camp with Hamas, Hezbollah and Osama bin Laden.
"Some seem to believe that we should negotiate with terrorists and radicals, as if some ingenious argument will persuade them they have been wrong all along. We have heard this foolish delusion before," Bush said.
"As Nazi tanks crossed into Poland in 1939, an American senator declared: "Lord, if only I could have talked to Hitler, all of this might have been avoided." We have an obligation to call this what it is -- the false comfort of appeasement, which has been repeatedly discredited by history," he added.
Ahmadinejad has come under international criticism for saying that Israel should be "wiped off the map" and has questioned whether the Nazi Holocaust that killed 6 million Jews actually took place.
Iran also backs Hamas, whose charter calls for Israel's destruction.
Bush said letting Iran acquire nuclear weapons would be an "unforgivable betrayal of future generations."
"America stands with you in firmly opposing Iran's nuclear weapons ambitions," Bush told Israeli legislators on the second day of his visit to the Jewish state.
Bush was in Israel to celebrate its 60th anniversary and try to shore up the faltering Israeli-Palestinian peace process.
He used his speech to hammer home his view that democracy could prevail against extremism in the Middle East but reserved his strongest words for Iran, Israel's main foe in the region.
"Permitting the world's leading sponsor of terror to possess the world's deadliest weapon would be an unforgivable betrayal of future generations. For the sake of peace, the world must not allow Iran to have a nuclear weapon," Bush said.
Bush has led an international campaign to isolate Tehran diplomatically over its nuclear ambitions. He says there is a danger that Iran will use nuclear technology to build a bomb. Tehran says its programme is for peaceful civilian purposes.
Bush has accused Iran of fomenting violence against Israel by Palestinian militants, of using Hezbollah to try to destabilise Lebanon's elected government and of arming and training Shi'ite militias in Iraq. (Editing by Mark Trevelyan)
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