"There is no reason to doubt that it is real. US intelligence believes it is Bin Laden's voice," an official said.
In the message posted on an Internet website on Saturday, Bin Laden accuses the government of Prime Minister Nuri al-Maliki, a Shiite, of being "treacherous and apostate".
He also warns against deploying international forces in lawless Somalia and expressed backing for Islamist forces there.
Bin Laden says that a US-backed bid for the deployment of international forces in Somalia was part of a crusade to crush budding Islamic rule.
It calls on all Somalis to back the Council of Islamic Courts movement in its bid to build an Islamic state in Somalia.
Bin Laden posted another message on Friday in which he affirmed al-Qaeda’s campaign in Iraq despite the death of the group's head there, Abu Musab al-Zarqawi, in a June 7 US air attack.
Bin Laden is still believed to be hiding somewhere in Afghanistan although there has been no confirmed sighting of him for some time.