"You mujahedeen (holy warriors), Iraqis and women, increase your attacks on your enemies," the voice said. It resembled Saddam's voice, but there was no way to independently verify who was speaking.
The voice also called on Iraqis to protest against occupation in the streets, to "beat the walls in protest" and to donate money to the resistance.
The voice called on U.S. forces to withdraw from Iraq immediately.
"We call on you to withdraw your armies as soon as possible and without any conditions or restrictions, because there is no reason for you to suffer more losses, which will be disastrous for the Americans," the voice said, claiming the recording was made in "mid-September."
Meanwhile, the commander of the U.S.-led coalition in Iraq said in an interview published Wednesday that U.S. forces, already under pressure from a guerrilla-style resistance, now face revenge attacks from ordinary Iraqis angered by the occupation.
In other developments:
North of Baghdad, there were at least three separate attacks on U.S. forces with roadside bombs in less than 1½ hours Wednesday morning. Witnesses reported injured soldiers, but details were unclear. The attacks hit U.S. Humvees about 12 miles north of Baghdad near al-Taji.
While U.S. forces increasingly patrol Iraqi hotspots with American-trained local militiamen, citizens voice growing anger with tactics that are seen as heavy-handed and insensitive to Iraqi social and religious customs.
"We have seen that when we have an incident in the conduct of our operations, when we killed an innocent civilian, based on their ethic, their values, their culture, they would seek revenge," Lt. Gen. Ricardo Sanchez was quoted as telling The Times newspaper in London.
Coalition forces were seeking "to ensure that when a mistake has been made and when we have inadvertently wound up killing someone that we go and do the right thing culturally to take care of those families." The Times' report did not elaborate on those steps.
Sanchez's remarks came after the friendly fire killing late last week of eight Iraqi policemen by American soldiers near Fallujah, 30 miles west of Baghdad. The military and the U.S. administrator for Iraq, L. Paul Bremer, have apologized.
The last purported Saddam tape was broadcast on Sept. 1 by the Al-Jazeera television network, and the CIA said it was likely authentic. In that message, the speaker denied Saddam or his followers were behind a bombing days earlier on a Shiite Muslim shrine in Najaf that killed a leading cleric, Ayatollah Mohammad Baqir al-Hakim, and scores of others.