The Stuxnet computer worm caused relatively limited damage to Iran's nuclear programme and failed to stop the Islamic republic stockpiling enriched uranium, a US-based think-tank said in a report.

Stuxnet is believed to have knocked out in late 2009 or early 2010 about 1,000 centrifuges -- machines used to refine uranium -- out of the 9,000 used at Iran's Natanz enrichment plant, the Institute for Science and International Security (ISIS) said.

Security experts say Stuxnet may have been an attempt by Iran's enemies to sabotage the nuclear programme, which Western nations fear is intended to produce weapons despite Tehran's denials. The worm, which has been described as a guided cyber missile, possibly originated in Israel or the US.

"Although Stuxnet appears to be designed to destroy centrifuges at the Natanz facility, destruction was by no means total," ISIS experts David Albright, Paul Brannan and Christina Walrond wrote in the analysis dated Feb. 15.

"Assuming Iran exercises caution, Stuxnet is unlikely to destroy more centrifuges at the Natanz plant. Iran likely cleaned tbe malware from its control systems."

Enriched uranium can be used to fuel nuclear power plants, which is Iran's stated aim, or provide material for bombs if processed much further. Western powers accuse Iran, a major oil producer, of seeking to develop nuclear weapons capability.

Any setbacks in Iran's enrichment campaign could buy more time for efforts to find a diplomatic solution to its stand-off with world powers, even though talks in Geneva in December and Istanbul last month failed to bridge the gap.

Steady enrichment

 Israel and the US have not ruled out military action if diplomacy fails to resolve the dispute.

ISIS said cyber attacks such as Stuxnet were likely to continue in the absence of a negotiated settlement. "They provide an alternative to military strikes against Iran's known nuclear sites, a tactic that most see as likely to be ineffectual or counterproductive," the report said.

The cyber attack that is believed to have destroyed centrifuges at Natanz about a year ago "rattled the Iranians".

But Iran took steps that probably reduced further damage by Stuxnet and shut down many centrifuges -- finely calibrated cylindrical devices that spin at supersonic speed to increase the fissile element in uranium -- for months, ISIS added.

"While it has delayed the Iranian centrifuge programme at the Natanz plant in 2010 and contributed to slowing its expansion, it did not stop it or even delay the continued buildup of LEU (low-enriched uranium)," the report said.

Stuxnet did not lower Iran's output of refined uranium last year, even though it could help to explain why quantities did not increase significantly. "The relatively limited damage implies that destroying centrifuges through a cyber attack may be more difficult to do than commonly perceived," it added.

Iran's atomic activities have also been suffering from design-related technical problems and increasingly tough sanctions which make it more difficult for Tehran to acquire the equipment and other materials it needs for its enrichment work.

Despite such problems, diplomats and experts say Iran has resumed steady enrichment after a brief halt and that it has now has amassed enough LEU for one or two bombs if refined much further. – Reuters