The message, the second in as many days purportedly from the Western world's most wanted man, came as yet another sectarian car bombing shook Baghdad, killing 66 people in one of the worst attacks this year.

A White House official said the message, if authenticated by US intelligence services, reflected al-Qaeda's "dark vision" and shows that bin Laden and his supporters "offer nothing beyond further fighting, conflict and misery."

The speaker in the audio message posted on Saturday also warned against deploying international forces in lawless Somalia and expressed backing for Islamist forces there.

The voice said Iraqi Sunnis, once dominant under the ousted regime of Saddam Hussein, were being subjected to "a campaign of genocide at the hands of the gangs of grudge and treachery," a reference to Shias.