Dubai: 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 attack apparently affected 200,000 router switches across the world in a widespread attack, including 3500 switches in our country," Iran's Communication and Information Technology Ministry said in a statement on Saturday.

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.

A blog published on Thursday by Nick Biasini, a threat researcher at Cisco's Talos Security Intelligence and Research Group, said: "Several incidents in multiple countries, including some specifically targeting critical infrastructure, have involved the misuse of the Smart Install protocol ...

"As a result, we are taking an active stance, and are urging customers, again, of the elevated risk and available remediation paths."