Hackers have attacked networks in a number of countries including data centres in Iran where they left the image of a US flag on screens along with a warning: "Don't mess with our elections", the Iranian IT ministry said on Saturday.

"The attack apparently affected 200,000 router switches across the world in a widespread attack, including 3,500 switches in our country," the Communication and Information Technology Ministry said in a statement carried by Iran's official news agency IRNA.

The statement said the attack, which hit internet service providers and cut off web access for subscribers, was made possible by a vulnerability in routers from Cisco which had earlier issued a warning and provided a patch that some firms had failed to install over the Iranian new year holiday.

Iran's IT Minister Mohammad Javad Azari-Jahromi posted a picture of a computer screen on Twitter with the image of the US flag and the hackers' message. He said it was not yet clear who had carried out the attack.

Azari-Jahromi said the attack mainly affected Europe, India and the United States, state television reported.

"Some 55,000 devices were affected in the United States and 14,000 in China, and Iran's share of affected devices was 2 percent," Azari-Jahromi was quoted as saying.