c|net - The U.S. government warned American private financial services on Thursday of an al-Qaida call for a cyberattack against online stock trading and banking Web sites 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 U.S. 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 U.S. 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 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.