A person familiar with the warning said al Qaeda may be aiming to penetrate and destroy the databases of U.S. online stock trading and banking websites.

    The Department of Homeland Security confirmed an alert had been distributed but said there was no reason to believe the threat was credible.

    Homeland Security spokesman Russ Knocke said the U.S. Computer Emergency Readiness Team issued a "situational awareness report to industry stakeholders."

    The warning said the threat called for attacks to begin 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.

    Denial of service is when attackers try to prevent legitimate users from gaining access to a website. It can be done by flooding network connections or filling up storage space so the website becomes paralyzed.

    The warning said the threat came from a group calling itself "ANHIAR al-Dollar."

    A person familiar with the warning said the effort was related to al Qaeda and intended to avenge "Muslim brothers in the crusaders' Guantanamo prison camp."

    Robert Albertson, a chief investment strategist for 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.

    A government source said government regulators were being briefed on the warning.

    (Agencies)