A mysterious halt in Iran's uranium enrichment activities earlier this month, as revealed in a new report by the UN atomic watchdog, may have been due to a cyber attack, experts suggested Wednesday.

But the enrichment programme is vulnerable to wider technical problems -- particularly since it uses outdated technology -- as well as international sanctions, the experts argued.

A restricted report by the International Atomic Energy Agency, a copy of which was obtained by AFP, says no uranium-enriching centrifuges at Iran's enrichment facility in Natanz were being fed with nuclear material on at least one day this month.

Centrifuges are finely calibrated cylindrical devices that spin at supersonic speed to separate out an isotope of uranium used to make nuclear fuel or, if refined to a much higher degree, for the fissile material for an atomic bomb.

"On November 16, no cascades were being fed with UF6" or uranium hexafluoride, the nine-page report revealed, without offering any explanation as to a possible cause of the outage.

Uranium hexafluoride, a toxic gas, is the particular form of uranium used in the enrichment process.

Most of the machines at Natanz are used to produce low-enriched uranium or LEU, which is used to make nuclear fuel.

A much smaller pilot plant where earlier this year Iran began enriching uranium to 20 percent purity -- ostensibly to produce radioisotopes for medical purposes -- was not affected.

The IAEA's inspectors were not yet able to determine exactly how long the halt lasted: a few hours on November 16, an entire day or even longer.

The inspectors have not been back since, but Iranian authorities informed them that on November 22 nuclear material was being fed back into the centrifuges.

Iran quickly ruled out a possible cyber attack and denied that the enrichment work was experiencing technical problems.

But experts see as "feasible" the theory that the stoppage was caused by a computer worm called Stuxnet that has infiltrated Iran's nuclear facilities recently.

Israel is suspected of designing Stuxnet to sabotage its arch foe's nuclear programme, which the West believes is simply a cover to build an atomic bomb, though Iran denies it.

"It's certainly feasible, because of the kind of technical problems it can cause in the control systems for centrifuges," said Mark Hibbs, senior associate at the Carnegie Endowment for International Peace.

"Centrifuges are very sensitive machines and if you disrupt one of them, you can cause a large number of others to crash."

Analysts say Iran's enrichment equipment is even more vulnerable to a possible cyber attack because it is intrinsically flawed.

Most of the centrifuges, labelled as P-1 or IR-1, are based on a model dating back to the 1970s that is extremely prone to breakage.

"It's very likely that the halt was in connection with efforts to overcome chronic technical problems," said Mark Fitzpatrick of the International Institute for Stragegic Studies.

"The Stuxnet malware appears to have contributed to these problems, but the P-1 centrifuges have intrinsic problems anyway."

Hibbs at Carnegie suggested Iran could soon abandon the P-1 or IR-1 centrifuge in favour of more advanced and more reliable models.

According to the IAEA report, there are 8,426 IR-1 centrifuges currently installed at Natanz -- down from 8,856 at the end of August -- with only just over half of those, or 4,816, being fed with nuclear material.

Significantly, Iran told the IAEA it intends to reduce by a third the number of IR-1 machines at its second enrichment site at Fordow, near Qom, the existence of which Tehran only declared after it was discovered by western intelligence services.

David Albright of the Institute for Science and International Security saw that as a sign Iran had decided to downgrade the role of Fordow as an enrichment plant.

But Hibbs said: "We may be seeing the silent termination of the IR-1 problem" and a switch to more advanced models.

The challenge for the West was "to stop Iran's nuclear programme in its tracks or at least slow it down significantly, because Iran will find it difficult to procure the materials such as maraging steel and carbon-fibre needed" in the face of international sanctions, he added.