Organisations including the Iranian Ministry of Petroleum and the National Iranian Oil Company were the target of a computer worm attack on Sunday, according to the Mehr news agency.
"The cyberattack disrupted internet access at the aforementioned organisations and companies, and the relevant officials are investigating the issue," a Mehr article said on Monday.
Mehr reported that the worm was detected before it could infect systems.
However, the Iranian Ministry of Petroleum said in a statement on Monday that an attack had succeeded in stealing data from non-essential systems.
Iran disconnected its main oil terminal, Kharg Island, from the internet as a result of the cyberattack, AFP reported on Monday. Other facilities were also disconnected, said AFP.
In February 2011, ZDNet UK's sister site CNET News reported that Iran was probably the subject of the Stuxnet worm attack that targeted nuclear facilities.
The National Iranian Oil Company had not responded to a request for comment at the time of writing.