The International Atomic Energy Agency also said Iran's full cooperation in clearing up questions about its nuclear program is "overdue," as it outlined compliance with the IAEA's investigation since February 2003 into an Iranian civilian nuclear program which the United States claims hides covert weapons activities.

The IAEA said that as of last Tuesday "approximately 4,000 kilogramspounds) in the form of UOC (uranium yellowcake ore) had been fed into the process" at the uranium conversion facility in Isfahan, according to a copy of the confidential report made available to AFP.

Iran's resumption in early August of uranium conversion fuel work, which it had broken off last November to start talks with the European Union on guaranteeing its nuclear program is peaceful, has scuttled the negotiations and could lead to Iran being referred to the Security Council when the IAEA meets on September 19 in Vienna.

Iran has not however embarked on uranium enrichment, which takes the gas made from conversion and spins it through cascades of centrifuges to make the enriched uranium that can be fuel for civilian power reactors but also in highly enriched form the explosive core of atom bombs, the report said.

In Tehran, top nuclear negotiator Ali Larijani said the report contained criticism of the Islamic republic that was politically motivated, but nevertheless pledged Iran would continue to cooperate with the International Atomic Energy Agency (IAEA).

Larijani said the criticism of Iran in the report was "neither legal nor technical" -- repeating Iran's argument that as a signatory to the nuclear Non-Proliferation Treaty (NPT) it has the "right" to carry out fuel cycle work for peaceful purposes.

"We don't think this political part is important. We will continue our cooperation with the agency (IAEA) so that the small questions that are unresolved will be resolved," Larijani told state television.

Earlier he said the Iranian government needs time "to create a reasonable way to resolving the nuclear issue," he was quoted as saying by the student news agency ISNA.

European Union foreign policy chief Javier Solana said Thursday in Britain that the EU was "ready to go to the Security Council" for possible sanctions if Iran failed to heed an IAEA call to stop nuclear fuel work.

The IAEA's 35-nation board of governors had made the call in a resolution on August 11 and also charged IAEA chief Mohamed ElBaradei with reporting to it by September 3 on Iranian compliance.

US President George W. Bush said last month that "if Iran doesn't take the steps described in the resolution, we would expect that the next step would be referral to the Security Council."

The report said Iran was still refusing to allow its inspectors visits to the Parchin military site, where it has been to only one part of what is a huge complex where weapons explosive work is suspected, and to the Lavizan site in Tehran where there is suspicion that enrichment work was carried out and where the soil has been razed.

There are also open questions about Iran's work with both uranium and plutonium, the two raw materials for atom bombs, as well as with centrifuges, especially the sophisticated P-2 model, and about sensitive materials Iran has tried to obtain on the black market such as beryllium metal which can be used to make the trigger for nuclear weapons.

Since the IAEA "is not yet in a position to clarify some important outstanding issues after two and a half years of intensive inspections and investigation, Iran's full transparency is indispensable and overdue," the IAEA said.

"The burden of proof is back to Iran," a senior official close to the IAEA said.

ElBaradei has said his agency has so far confirmed Iranian declarations about its nuclear activities but that this does not mean there are not undeclared activities and that "the jury is still out" on whether Iran's program is peaceful.

IAEA inspections into traces of weapons-grade uranium found in Iran largely support Tehran's claim that the particles came from imported equipment but more research is needed to fully clear Iran of suspicions of making uranium itself, Friday's report said.

This was not quite the vote of confidence Iran had hoped for from the contamination inquiry.

The investigation tends "on balance to support Iran's statement about the foreign origin of most of the observed highly enriched uranium contamination," the report said, after Pakistan had given the IAEA centrifuge parts to compare with identical components Iran had bought from an international smuggling ring run by disgraced Pakistani scientist Abdul Qadeer Khan.

But "it is still not possible at this time, however, to establish a definitive conclusion with respect to all of the contamination, particularly the LEU (low-enriched urnanium, below weapons-grade levels) contamination," the report said, calling for Iran to provide timetables of its centrifuge work.

The IAEA also said Iranian experiments with plutonium, another atom bomb material, were still being investigated.

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