The US government has put out an alert over a possible online attack by al Qaeda.

The warnings were circulated to private financial institutions about a possible attack that is designed to wipe out their databases.

They follow a call from the group ANHIAR al-Dollar to attack financial firms throughout the month of December, with action supposedly beginning today.

The terrorist group said the attacks would be a response to the US policy of keeping suspects at Guantanamo Bay.

The attacks would avenge "Muslim brothers in the crusaders' Guantanamo prison camp," a source who received the warning told Reuters.

A spokesman for the Department of Homeland Security said that any attacks would most likely come in the form of denial of service.

However, the agency was keen to downplay the seriousness of the threat.

The US Computer Emergency Readiness Team, part of Homeland Security, confirmed that it had sent out the alert as a precautionary measure, but said it didn't believe the threat was credible.