Iran said it had detected the Duqu computer virus that experts say is based on Stuxnet, the so-called "cyber-weapon" discovered last year and believed to be aimed at sabotaging the Islamic Republic's nuclear sites.
The head of Iran's civil defence organisation told the official IRNA news agency that computers at all main sites at risk were being checked and that Iran had developed software to combat the virus.
"We are in the initial phase of fighting the Duqu virus," Gholamreza Jalali, was quoted as saying.
"The final report which says which organisations the virus has spread to and what its impacts are has not been completed yet.
"All the organisations and centres that could be susceptible to being contaminated are being controlled," he said.
News of Duqu first surfaced on October 18 when Symantec said in a report that a research lab with international connections had alerted it to a mysterious computer virus that "appeared to be very similar to Stuxnet".
While Stuxnet was aimed at crippling industrial control systems and may have destroyed some of the centrifuges Iran uses to enrich uranium, experts say Duqu appeared designed to gather data to make it easier to launch future cyber attacks.
"Duqu is essentially the precursor to a future Stuxnet-like attack," Symantec said.
Instead of being designed to sabotage an industrial control system, the new virus is designed to gain remote access capabilities, it said.
Iran said in April it had been targeted by a second computer virus which it identified as "Stars". It was not immediately clear if Stars and Duqu were related but Jalali described Duqu as the third virus to hit Iran.
Tehran said Stuxnet had not inflicted serious damage before it was detected and blamed the United States and Israel for the virus which appeared to be aimed at crippling the nuclear programme they say is aimed at making atomic weapons, a charge Iran denies.
The International Atomic Energy Agency issued a report last week that contained what it called credible evidence pointing to military dimensions to Iran's atomic activities, fueling demands in Washington and Europe for further sanctions.