The report is due in mid-May, possibly next week, and comes as International Atomic Energy Agency inspectors are completing months of investigations into US allegations that Iran is secretly developing nuclear weapons.

The IAEA is to hold a meeting of its 35-nation board of governors in Vienna on June 14.

Reflecting the current thinking of investigators, one diplomat said that if the Iranians "weren't working on something that hasn't been declared, the contamination should be evenly spread throughout Iran's nuclear installations."

Radioactive dust spreads in a uniform pattern but particles of highly enriched uranium have been found in specific sites, hinting that "someone brought material or equipment and then removed it."

IAEA inspectors have reported two such concentrations -- at a Kalaye Electric Company workshop in Tehran and at the Natanz pilot fuel enrichment plant 250 kilometres (150 miles) south of the Iranian capital.

The diplomat refused to confirm if other sites had been found but said that the Kalaye company had many sites in Tehran and throughout Iran.

Another diplomat confirmed that contamination had been found at other sites but downplayed the importance of this.

"They've moved equipment and we find what we expect to find. Unfortunately we don't learn much about it," he said. He did not provide details.

Highly enriched uranium (HEU) can be the raw material for a nuclear bomb, with weapons-grade uranium enriched to over 80 percent of the U-235 isotope, usually by cascades of centrifuges.

IAEA chief Mohamed ElBaradei said in April that no traces of HEU have been found beyond the two sites at Kalaye and Natanz.

The traces found at "one room in the Kalaye Electric Company workshop" were particles of "uranium enriched to 36 percent," according to an IAEA report on Iran filed in February.

Iran has been asked to explain this "particularly in light of its declaration that it has not enriched uranium to more than 1.2 percent U-235 using centrifuge technology," the report said.

Nuclear expert David Albright told AFP from Washington that one would only try to take 36-percent enriched uranium and enrich it further in the framework of a weapons program.

He said this could be done in "small experiments" to test centrifuges to know how many would be needed in an cascade of centrifuges to carry out enrichment.

IAEA inspectors are looking for other sites where there is such telltale 36-percent enriched contamination and have been systematically visiting "places that have to do with Iran's nuclear program," including dual-use facilities not obviously related to suspect atomic weapons work, the first diplomat said.

The Iranians have said the contamination is from imported equipment, with Pakistan believed to be a source for such equipment through an international black market.

But the diplomat said that equipment imported from Russia or fuel used in IRT research reactors was likely to be the source for this particular kind of enriched uranium.

The diplomat said the Iranians were "surprised" about the thoroughness of the IAEA quest, which has included visits to factories where machines are made that balance the rotors in centrifuges, special furnaces used to make uranium metal and sites where mass spectrometers are in operation.

Last October, Iran gave the IAEA what it said was a complete declaration of its nuclear activities.

It was later found to have made a number of omissions, including its acquisition of designs for sophisticated P-2 centrifuges.

Iran is now to deliver a further report under the provisions of an additional protocol it signed in December to the nuclear Non-Proliferation Treaty (NPT).

This report should be more complete than the one in October since the additional protocol mandates a tougher inspections regime.

IAEA inspectors will then file their own report to the IAEA board ahead of its meeting in June.

WAR.WIRE