Catherine Ashton's language was unusually forthright for the West's chief negotiator over Iran's nuclear program.
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UN chief condemns Iran leaders' anti-Israel remarks
Ashton "strongly condemns the outrageous and hateful remarks threatening Israel's existence by the Supreme Leader and the President of the Islamic Republic of Iran," said a statement by her spokesman.
"Israel's right to exist must not be called into question."
On Friday, Ahmadinejad told demonstrators in state-organized protests that "in the new Middle East ... there will be no trace of the American presence and the Zionists". As thousands of Iranians shouted "Death to America, death to Israel", Ahmadinejad called Israel a "cancerous tumor" for its occupation of Palestinian land.
Earlier this week Iranian media reported that Iran's Supreme Leader Ayatollah Ali Khamenei had said Israel would one day be returned to the Palestinian nation and would cease to exist.
Ashton is acting as chief negotiator for six powers - the United States, Russia, China, France, Germany and Britain - that are trying to persuade Iran to scale back its nuclear program through economic sanctions and diplomacy. They fear Iran's nuclear program aims at producing weapons, though Tehran says it serves peaceful purposes only.
Ashton and Iran's chief negotiator agreed at the start of August to hold more talks about Iran's nuclear work, but there has been no sign of imminent progress in the decade-long dispute.
Ashton "calls upon Iran to play a constructive role in the region and expects its leaders to contribute to de-escalate tension and not to fuel it", Saturday's statement said.
On Friday, UN Secretary-General Ban Ki-moon said the verbal attacks on Israel were "offensive and inflammatory."
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