Hackers from Iran may have targeted the three biggest U.S. banks as part of a broader cyber offensive against the United States.

Bank of America (BAC), JPMorgan Chase (JPM) and Citigroup (NYSE: C) have endured repeated attacks from Iranian hackers since last 2011, Reuters reported Friday. The digital campaign reportedly has sought to disrupt the banks' websites and corporate networks by flooding them with traffic.

Neither the banks nor the New York office of the FBI commented on the report, which comes amid warnings by both industry and law enforcement groups that urge financial institutions to remain vigilant.

Bank of America and Chase said this week that some customers who tried to log on to their websites encountered delays, although both companies say service has resumed normally.

On Wednesday, the Financial Services Information Sharing and Analysis Center, an industry group, raised its cyber threat level to "high" from "elevated" and urged members to monitor their networks carefully. The FBI warned Monday that thieves may be attacking financial firms' websites to steal information that could be used to make fraudulent wire transfers.

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