Iran warned United Nations Secretary-General Ban Ki-moon against "double standards" on Thursday after he deplored anti-Israeli comments by Iranian President Mahmoud Ahmadinejad at a UN conference this week.

In a letter to Ban, Iranian UN Ambassador Mohammed Khazaee denounced the furious reactions to Ahmadinejad's speech at a time when, he said, UN officials were silent on Israel's treatment of Palestinians.

On Monday, the Iranian president, who has questioned the Nazi Holocaust, condemned Israel at a UN conference in Geneva on racism as a "totally racist government" founded "on the pretext of Jewish sufferings."

In an unusual criticism of a head of state, Ban deplored Ahmadinejad's use of the conference platform "to accuse, divide and even incite."

Khazaee implied the comment contravened the need for a UN chief to be "impartial and fair."

"Utmost vigilance should be exercised in order to prevent any situation where the United Nations and its distinguished officials would be seen as applying selective approaches, practicing double-standards or taking biased positions," he said.

"All these incomprehensible reactions have been made against the statement of [Ahmadinejad] while the United Nations and its officials have remained silent on [the] Israeli regime's abhorrent crimes against the innocent Palestinians, and on the threats, allegations and distortions made against... Iran by the Israeli regime," Khazaee added.

The Iranian envoy was referring to Israeli statements that Iran's nuclear program, which Tehran says is peaceful, threatens its existence and to speculation in Israel about a possible strike on Iran's nuclear facilities.

Khazaee also condemned a walkout by European Union delegates to the Geneva conference during Ahmadinejad's speech, calling it a "manifestation of intolerance" in disregard of U.N. principles of freedom of expression.

Ahmadinejad, the Iranian letter said, had been "subjected to unfair and unwarranted harsh criticism only for having tried to pronounce the positions of the country that he represents."

The Iranian president, it said, had simply expressed "what, in our view, constitutes the position and the concern of the overwhelming majority of the UN member states on the plight of the Palestinian people caused by the Israeli regime's policies and practices."