The United States government warned American private financial services on Thursday of an Al-Qaeda call for a cyber attack against online stock trading and banking websites beginning on Friday, a source said.
The source, a person familiar with the warning, said the Islamic militant group aimed to penetrate and destroy the databases of the US financial sites.
The Department of Homeland Security confirmed an alert had been distributed but said there was no reason to believe the threat was credible.
The US Computer Emergency Readiness Team issued a "situational awareness report to industry stakeholders," said Homeland Security spokesman Russ Knocke.
The warning said the threat called for attacks will begin on Friday and run through the month of December in retaliation for the United States keeping terrorism suspects at the Guantanamo Bay naval base in Cuba.
"Denial of service is what it called for," said a Homeland Security official who spoke on condition of anonymity.
A person familiar with the warning said the threat came from a group calling itself "ANHIAR al-Dollar."
The effort was related to Al-Qaeda and intended to avenge "Muslim brothers in the crusaders' Guantanamo prison camp," the source said.
Reaction in the financial community was muted, with markets showing little or no reaction.
New York Republican Rep Peter King, chairman of the House of Representatives Committee on Homeland Security, said the report was "nothing to panic over, but it will be looked at very carefully."
Robert Albertson, chief investment strategist at Sandler O'Neill & Partners in New York, said it was unlikely Al-Qaeda members could do serious harm to financial websites.
"I'm not saying there aren't precautions to be taken, but I just can't fathom how there would be serious havoc," he added.
Brian Jenkins, a terrorism expert with the RAND Corp, said that such threats were not unusual.
"There is a regular stream of Jihadist exhortations to attack various targets," he said.
"Financial organisations stay at a fairly high level of readiness anyway because of regular assaults."
A government source said regulators were being briefed on the warning.