The Iranian oil ministry said on Tuesday its IT systems had suffered no lasting damage from a suspected cyber attack, but its experts would require two or three days to investigate and address the impact of the virus.
The virus hit the internet and communications systems of the oil ministry and national oil company late on Sunday, forcing Iran to disconnect the control systems of Kharg Island, which handles the vast majority of Iran's crude exports, and a number of other oil facilities.
"Fortunately, because of the rapid measures taken by our experts, this ministry has sustained no damage to its computer data," the head of the ministry's civil defence team, Hamdullah Mohammadnejad, said.
Iranian state news agency IRNA quoted him as saying the cause of the problem was being identified and it would take two to three days for the issue to be resolved.
"All units with the oil industry back up their data on a daily and long-term basis. But in cases where information has been impaired to any extent, the backup data is being replaced," he added.
The oil ministry set up a crisis unit and disconnected IT systems at its headquarters, the national oil company and at its oil terminals but said all operations units continued to function normally. An industry source said oil was being loaded at Kharg island on Tuesday.
"Fortunately our international oil selling division has not been affected," IRNA quoted a senior ministry official as saying in an earlier report. "There is no panic, but this shows we have shortcomings in our security systems."
The virus is likely to draw comparisons with the Stuxnet computer worm which affected Iranian nuclear facilities in 2009-10.
Iranian officials have accused the United States and Israel of trying to sabotage its nuclear programme through viruses like Stuxnet.
Security specialists say the latest problems in Iran's IT systems could be an attempt to impair Iran's ability to trade in oil, or might even have been a technical failure.
The United States and its allies have imposed increasingly tough sanctions against Iran's oil industry over its nuclear programme, which they believe is geared towards producing nuclear bombs. Iran says the nuclear programme is for peaceful purposes only.
EU member states have significantly reduced any orders of Iranian oil in anticipation of a total ban set to be implemented across the European Union in July.