Local news agencies reported on Monday that a virus had struck the computer and communication systems of Iran's main oil export facilities on Kharg Island as well as the internal network and the websites of its oil ministry and subsidiary organisations.
The semi-official Mehr news agency quoted ministry officials as saying an investigation was under way. "We are making plans to neutralise this cyber-attack," said the deputy oil minister in charge of civil defence, Hamdollah Mohammadnejad.
The Kharg Island oil terminal, which exports 80% of the country's daily 2.2m barrels, was hit by the virus, along with terminals on the islands of Gheshm and Kish.
Officials told Mehr that disconnecting the ministry from the global internet was a precautionary move to protect its main services and denied it was taken offline because of damage caused by the virus. Among services provided by the ministry's website are fuel cards, which millions of Iranians use on a daily basis to buy petrol for cars.
No individual or group has yet claimed responsibility for the attack and the motives behind it are still unknown, but other Iranian energy sectors have also been targeted by similar cyberstrikes in recent years.
In 2010 a computer worm called Stuxnet, which was believed to have been designed to sabotage the country's enrichment of uranium, hit many Iranian nuclear sites.
Unlike this time, officials initially denied nuclear facilities had been infected by Stuxnet but later admitted it had happened, claiming they had neutralised it before it inflicted any damage.
Iranian officials accuse the west, the US and Israel in particular, of being behind a covert campaign to stop Iran from advancing in its nuclear activities by using cyber-attacks and murdering its nuclear scientists, four of whom have been killed in the past two years.
Iran's response to the cyber-attacks has been to work on a countrywide network project, called national internet, whose primary aim is to protect Iranian military, banking and sensitive data from the outside world but also aims to be a substitute for the world wide web for ordinary users. The plan, which has not been launched yet, has drawn a great deal of criticism among Iranian web users.
An Iranian IT expert involved in Iran's national internet project, who spoke to the Guardian on condition of anonymity earlier this year, said: "Iran has fears of an outside cyber-attack like that of the Stuxnet, and is trying to protect its sensitive data from being accessible on the internet [by creating a secure intranet]."
