WASHINGTON - The US government has warned US private financial services of an al Qaeda call for a cyber attack against US online stock trading and banking Web sites beginning Friday, officials said on Thursday.

An image taken from an Internet posting by al Qaeda's media arm, al Sahab on September 11, 2006, shows Osama bin Laden speaking in an unknown location. Bin Laden may be the leading symbol of global Islamist militancy but the al Qaeda leader wields less influence over Islamist ideology than more obscure religious thinkers, according to a new study issued on Wednesday. [Reuters]

The officials -- a person familiar with the warning and a spokesman for the Department of Homeland Security -- said the Islamic militant group aimed to penetrate and destroy the databases of the US stock market and banking Web sites.

Homeland Security said it had no evidence to corroborate the threat but had issued the warning out of an "abundance of caution." The department said in a statement that the threat was for all of December.

"There is no information to corroborate this aspirational threat. As a routine matter and out of an abundance of caution, US-CERT issued the situational awareness report to industry stakeholders," said Homeland Security spokesman Russ Knocke.

US-CERT is the US Computer Emergency Readiness Team. The US government said the threat was to avenge the holding of suspected terrorists at the US prison in Guantanamo.