Dan Gillerman also protested against what he called "demented" remarks by Iranian President Mahmoud Ahmadinejad, who called Israel a "dirty microbe" and a "savage animal."

"Our expectations, and those of many of my colleagues at the United Nations, are not high," Gillerman was quoted on public radio as saying in New York late on Wednesday.

"I fear that once again we will be disappointed by this report," he added.

International Atomic Energy Agency (IAEA) chief Mohamed ElBaradei is due to issue a report in the coming days detailing Iran's cooperation with the UN nuclear watchdog.

"In recent years, Mohamed ElBaradei has demonstrated incomprehensible and exaggerated forbearance regarding Iran's race toward arms," Gillerman said.

"He systematically tries to sympathise with the Iranians and to minimise things. Our experiences with his previous reports have not been positive, and I do not expect any change."

Iran's refusal to halt uranium enrichment has already seen it slapped with two sets of UN Security Council sanctions, and the United States is pressing the body to agree a third package of measures.

However it appears that Tehran, which insists that its atomic drive is entirely peaceful, has been accelerating its nuclear programme despite the international pressure.

Diplomats at IAEA headquarters in Vienna said Iran is pushing ahead with tests of more advanced P2 centrifuges, which are more efficient than the P1 or first-generation machines used until now at Iran's nuclear facility in Natanz.

Regarding Ahmadinejad's remarks on Wednesday, Gillerman said: "I have told the secretary general of the United Nations that these are demented remarks that are reminiscent of Nazi propaganda.

"The international community should be warned because these threats against Israel, which are also threats against the entire world, should be taken seriously."

UN chief Ban Ki-moon later said it was "undesirable for the head of state of a member state of the United Nations to use such kind of language against any other member state."

Speaking at a rally in Iran, Ahmadinejad had said that "world powers have created a black and dirty microbe named the Zionist regime and have unleashed it like a savage animal on the nations of the region."

On Monday, the commander of Iran's Revolutionary Guards, Mohammad Ali Jafari, predicted that the Lebanese Shiite militant group Hezbollah would soon destroy Israel.

The Iranian president has provoked international outrage by repeatedly predicting that Israel is doomed to disappear. He also courted more controversy by playing down the scale of the Holocaust.