US companies have been alerted to possible terrorist plans to wipe out their databases by the US government.

Private financial companies are the targets, following calls from Al-Qaeda associated group, ANHIAR al-Dollar to attack the sector during December.

The part of US Homeland Security responsible for cyber-crime, the US Computer Emergency Readiness Team has confirmed it sent out an alert warning of an online attack, but added the credibility of the threat was questionable.

A Department of Homeland Security spokesman said denial of service attacks were the most likely form of threat.

Any attack was supposed to avenge "Muslim brothers in the crusaders' Guantanamo prison camp," a source recipient of the warning told Reuters.