Iran said on Saturday it had evidence Washington was behind the latest killing of one of its nuclear scientists, state television reported, at a time when tensions over the country's nuclear program have escalated to their highest level ever.

U.S. Secretary of State Hilary Clinton denied responsibility and Israeli President Shimon Peres said Israel had no role in the attack, to the best of his knowledge.

"We have reliable documents and evidence that this terrorist act was planned, guided and supported by the CIA," the Iranian foreign ministry said in a letter handed to the Swiss ambassador in Tehran, state TV reported. The Swiss embassy represents U.S. interests in a country where Washington has no diplomatic ties.

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State TV said a "letter of condemnation" had also been sent to Britain, saying the killing of Iranian nuclear scientists began after the head of Britain's MI6 spy service announced intelligence operations against states seeking nuclear weapons.