On a day when American forces killed 11 Iraqis in a firefight north of Baghdad — and a day after 28 United States soldiers were wounded, including 18 in a mortar attack on a military base — the speaker urged Iraqis to continue resisting the American and British "infidel invaders." 

Parts of the tape were broadcast on the Arab satellite channel Al Jazeera a day after the Bush administration offered a $25 million reward for information leading to Mr. Hussein's capture or evidence proving that he is dead. Lesser rewards of $15 million each were offered for the same information about his sons, Uday and Qusay.

On the tape, the voice exhorts Iraqis to protect "heroic fighters" who have staged attacks on American and British forces, leaving at least 25 Americans dead since May 1. The voice, which said the tape was made on June 14, warned that "the coming days will, God willing, be days of hardship and trouble for the infidel invaders." 

There was no way of knowing whether the tape was authentic. The White House spokesman Ari Fleischer, traveling with President Bush for an Independence Day speech in Ohio, said the tape would be "under review" by intelligence agencies.

The appearance of the audiotape, if authenticated as carrying Mr. Hussein's voice, would be a blow to the allied military effort, which has sought to portray Iraqi resistance as leaderless at the national level, scattered and ineffective. 

In the recording, the voice makes reference to operations that must be safeguarded and to "jihad cells" and "brigades" that have been formed. It urges Iraqis to avoid cooperating with the occupation authorities and suggests that plans have been laid for future operations in coming days.

"Make the mujahedeen secure and catch any spies," the voice said. "We call on Iraqis who deal with Americans to stop doing so." And the voice makes reference to the current debate over the justification for the war: "I ask the invaders, where are these weapons of mass destruction?" 

The broadcast of the tape came as hostilities continued between allied forces and members of the Iraqi resistance. Early today, American military officials said, soldiers from the Third Infantry Division's Seventh Cavalry killed 11 Iraqis as the Iraqis tried to ambush them on Highway 1 near Balad. "The attackers attempted to engage the patrol with small-arms fire and rocket-propelled grenades, but were all killed when the patrol returned fire," a military statement said. No American injuries were reported.

A soldier of the First Armored Division was shot and killed Thursday night while standing at his post outside the Iraq Museum on a day in which it had reopened its doors for an exhibition of gold and jewelry from Nimrud, the capital of the Assyrian Empire in 900 B.C. "The soldier was in the gunner's hatch" of a Bradley Fighting Vehicle "when he was hit," a military statement said. 

United States military officials have said there is no evidence of central "command and control" for the attacks that have plagued allied forces, though they acknowledged this week that there might be "regional" coordination. But authenticating the tape would indicate that Mr. Hussein was seeking to project himself, perhaps from the isolation of his hiding place, as the leader of a resistance movement.

The voice said, "I am with some of my companions in Iraq," and called upon Iraqis to "protect these heroic fighters and not give the invaders any information about them or their whereabouts."

A number of Iraqis who heard the recording today appeared convinced of the tape's authenticity. They said it was partly the timbre of the voice, but more the histrionics of the message, the classical Arabic idioms and his familiar refrain of Allah-u Akbar, "God is great," that constituted for them the vocal signature of the fallen Iraqi leader. 

Yet many Iraqis appeared unimpressed that Mr. Hussein had perhaps managed to broadcast his voice for the first time since he fled after a final appearance, still contested by some experts, at a public square in the Adhamiya neighborhood of Baghdad. "The tape of course is authentic," said Hathem Ali al-Ansaari, a money changer in the Karada district of Baghdad. "He spoke about resistance, we know that he is there. All these attacks, they are well prepared and synchronized."

Salem Kudish, 50, a grocery shop owner, said: "It's over, it's over, the people won't accept him coming back. I expect he is there, because without him we wouldn't have all these incidents, but it is useless."

Still others took great hope from the recording. North of Baghdad, near the American base where the 18 soldiers were wounded Thursday night, two of them seriously, a young man who refused to give his name claimed to be part of the resistance.

"Do you think if we continue to attack the Americans, they will leave in a few days?" he asked. He added that it was futile for American troops to search for Mr. Hussein. "Do you think any Iraqi would turn him over to the Americans? I wouldn't do that. If he was in my home, we would not turn him over," he said, even for the $25 million reward.

A spokesman for Al Jazeera said the 20-minute tape had been offered to the station by a caller this afternoon. It was transmitted by telephone and then edited for broadcast, the spokesman said.

One reference on the tape asserts that "the casualty numbers that the Americans are announcing are false." This statement was echoed on the streets of Falluja, west of Baghdad today, where a number of residents were trafficking in rumors that the United States military was dumping the bodies of dead American servicemen in the desert near the Syrian border to hide casualties. 

While military officials dismiss such rumors as absurd, the tales seem to be evidence of a systematic disinformation campaign orchestrated by Mr. Hussein's loyalists to portray American resolve as weaker than it appears in the statements of Mr. Bush and his commanders.

Mr. Hussein's message, if not his leadership, resonated in a number of mosques in central Iraq today.

A leader of Friday Prayers at Al Samarai mosque in Falluja told the faithful that God would exact revenge on any occupying army that "defiled" the rights of the people. He drew on the biblical example of a vengeful god that destroyed the army of Egypt under the pharaoh in the time of Moses.

"If all the armies came, we will still have the might go with us who is our army and God can't be frightened by the armies of the world," the prayer leader, who was not identified, said. "You Americans can't frighten us with your armies or your muscles nor your weapons and missiles because God is able to kill you as he killed the pharaoh."