Al-Jazeera television has broadcast what it says is a recent recording of former Iraqi President Saddam Hussein.
The voice urged Iraqis to resist occupying forces
The Arabic satellite channel said the taped message was dated 14 June, six weeks after US President George W Bush declared major hostilities over.
The speaker purported to be Saddam Hussein said that he was alive and well and living among Iraqis.
The broadcast comes as the top US official in Iraq, Paul Bremer, announced a $25m reward for information leading to the capture of Saddam Hussein.
A $15m reward was also being offered for similar information about the former president's two sons, Uday and Qusay.
The BBC's Frank Gardner says the voice on the tape sounds like Saddam Hussein - but that even if it proves not to be him its psychological effects will harm the coalition's efforts.
He says the tape's message is designed to rally those loyal to the old Baath party, and convince Iraqis that Saddam Hussein will one day return.
Al-Jazeera's chief editor Ibrahim Hilal said the tape was delivered to the station via telephone on Friday.
"Someone called us and played back the tape for us and we recorded it. It ran for over 20 minutes, but only 10 minutes are newsworthy.
"We don't know the source, or where the call came from. We have no reason to doubt its authenticity," he said.
US to analyse voice
The voice on the audio tape played by al-Jazeera says: "I tell you that I miss you, miss you, o beloved people, even though I am among you and in your ranks."
It says "jihad cells made up of Iraqi male and female fighters have been formed on a large scale" throughout Iraq to fight US-led coalition forces occupying the country.
He continued: "I call on you to provide a cover for the heroic mujahideen and not to give the infidel invaders and their collaborators any information about them, their activities or their whereabouts."
US officials said the Central Intelligence Agency would begin a detailed analysis of the tape to match it against known samples of Saddam Hussein's voice.
"We don't know if he's alive or dead. Regardless, he's out of power," said White House spokesman Ari Fleischer.
The Americans have been facing armed resistance in Iraq, with at least 19 US troops killed in attacks since 1 May.
US troops are engaged in a series of large military operations in the area to root out supporters of the ousted president who have been staging increasingly bold attacks on American forces.
On Thursday, a sniper killed a US soldier guarding the Iraqi National Museum in Baghdad. On Friday 18 soldiers were injured when mortar bombs were fired at one of the largest US military bases in Iraq, also near Balad.
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A few hours later US troops killed 11 Iraqis who ambushed their convoy north of Baghdad using rocket-propelled grenades and small arms fire, a US military spokesman has said.
The whereabouts of the former Iraqi leader himself have been a mystery since the fall of Baghdad on 9 April.
The date given for this recording is four days before US troops attacked a convoy of vehicles believed to have been carrying Saddam Hussein and one of his sons.