The work, involving the first stages of the nuclear fuel cycle, had sparked concern that Iran was violating its pledge to suspend nuclear fuel processing that could be used to make atomic weapons.

The freeze, which Tehran agreed to in a deal with the European Union, began on November 22. But Iran needed to finish processing uranium ore that had been introduced into fragile uranium conversion machines ahead of the suspension.

This work ended the week of February 14, a diplomat close to the UN nuclear watchdog International Atomic Energy Agency (IAEA) told AFP.

IAEA chief Mohamed ElBaradei is to issue a full report on the matter when his agency's 35-nation board of governors meets in Vienna on Monday to review how to proceed on Iran.

The European Union, whose deal held out the promise of incentives to Tehran, has undertaken talks on offering Iran trade, technology and security rewards in return for its freeze of all uranium enrichment activities.

However, concern over possible violations remains high.

The United States is warily watching the freeze -- designed to allay fears Tehran might be secretly making nuclear weapons -- and the Iran-EU negotiations.

Washington has charged that Iran is using the suspension to gain time to develop nuclear bombs and would like to see Tehran brought before the UN Security Council for possible sanctions.

The IAEA is monitoring the enrichment suspension.

"Iran stopped making UF4 (uranium tetrafluoride) a week ago," said the diplomat, who asked not to be named. UF4 is a uranium powder that is a key first step in the enrichment process that can make nuclear weapons.

Iran was processing a total of 37 tons of uranium yellowcake ore into UF4, the precursor to UF6 gas. That gas is then fed into centrifuges to filter out enriched uranium which is fuel for nuclear reactors but also, in highly refined form, the explosive core of atomic bombs.

The yellowcake makes what could be enough highly enriched uranium for three atomic bombs, experts said.

A second diplomat close to the agency said the IAEA has "under seal all the UF4 that was produced at the facility" in Isfahan where the Iranians perform uranium conversion.

The diplomat said a "clean-up operation" still has to be finished, getting contamination out of pipes for instance, in order to see exactly how much of the uranium powder was made.

Making the UF4 powder is as far as Iran can go in enrichment, according to the November agreement with the EU, which was endorsed by the IAEA.

Iran caused an uproar when it pushed ahead with the processing in last-minute activity before the suspension kicked in on November 22.

ElBaradei said Iran had produced ahead of the start of the freeze "3.5 tonnes of UF6 gas," not enough to make enough enriched uranium for even one bomb.

There are so far no reports of Iran continuing to produce UF6 gas.

Iran was merely processing what remained of the 37 tons of yellowcake it had got ahead of the start of the freeze into the Isfahan plant, diplomats said.

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