US Bancorp, JPMorgan Chase, Bank of America, PNC, and SunTrust have been selected as targets by the Cyber Fighters of Izz ad-din Al Qassam, who succeeded in knocking out more than a dozen US bank sites in October.

In a statement posted on Pastebin, the anonymous hackers promise that in the second phase of the so-called Ababil operation "the wideness and the number of attacks will increase explicitly; and offenders and subsequently their governmental supporters will not be able to imagine and forecast the widespread and greatness of these attacks".

In October, the US Financial Services Roundtable's technology unit, Bits, urged banks to review their strategies for patching IT systems, scan networks and manage bandwidth to minimise the number of non-attack related issues.

"They should also coordinate with ISPs and other service providers to implement controls such as scrubbing, rate limiting and source blocking," says Bits. "External-facing assets and applications should be assessed and communications strategies reviewed to include incident notification for both internal and external parties."