A man believed to be Usama bin Laden, in an audiotape broadcast Tuesday night on Al-Jazeera satellite TV, called on Muslims throughout the world to unite behind Iraq.

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The Al Qaeda leader also gave a vivid description of the shelling he and his followers took in Afghanistan after the Sept. 11 attacks, and told his followers:

"If we are in the trenches, they cannot get us. The enemy is going to waste their ammunition for nothing."

"Anyone who helps America, from the Iraqi hypocrites [opposition] or Arab rulers ... whoever fights with them or offers them bases or administrative assistance, or any kind of support or help, even if only with words, to kill Muslims in Iraq, should know that he is an apostate."

Earlier in the day, Secretary of State Colin Powell told a Senate committee that the audiotape would be aired, and that U.S. officials believe the voice on the recording is bin Laden's.

U.S. officials told Fox News after the airing of the tape that a conclusive analysis could take as long as 48 hours, but the voice on the tape was almost certainly bin Laden's.

They said they were operating under the assumption that the terror leader is still alive. One said, "This would appear to confirm that."

Another said, "Folks who make a living listening to this guy say this certainly sounds exactly like him."

Officials note that the speaker used the opportunity not only to tell Muslims to help defend Iraq but also to "whip up the troops -- to give them yet another reason to keep up the fight."

On the tape, bin Laden says the U.S. is waging a psychological war against Iraq. And he says the Iraqis should expect a massive bombardment from the air.

"We would like to confirm at this time the lies of America and their allies and what they are trying to do," he says on the tape. "We want you to be faithful in your fight, to believe in your God, the one and only God.

"Fight the agents of the Devil, because the Devil will be overcome and defeated. God will give us victory."

He also calls on Iraqis to carry out homicide attacks against Americans.

"We stress the importance of suicide bombings against the enemy, these attacks that have scared Americans and Israelis like never before," he says.

Bin Laden goes on to discuss the shelling Al Qaeda took in Afghanistan after Sept. 11, and says his survival is proof that Iraq will defeat America and its allies.

"It is going to be a great battle and we are going to defeat the enemy," he says.

"I am going to mention previous battles -- the trenches were long and we were hit with heavy bombardment in October 2001. But on the 13th of Ramadan, we experienced heavy shelling because the American leadership wanted to exterminate us. They were killing us every minute. The Air Force was flying overhead, and with all this shelling, we were able to defend this small piece of land. ...

"We were hit continuously with modern bombs, and ... we were able to hold our position and we kept repulsing them. They were going back carrying their dead.

"The Americans were unable to take over our position. They were unable to take that position from our small group of mujaheddin fighting in temperatures below 10 degrees. We lost many people, but they were accepted by God as martyrs. In that battle, a small number of warriors were unbeatable.

"So if people hold their positions ... and we hope our brothers in Iraq will do the same ... if we are in the trenches they cannot get us. The enemy is going to waste their ammunition for nothing."

Asked later Tuesday whether the latest message fuels speculation that bin Laden may still be alive, State Department spokesman Richard Boucher said, "We do worry about content of the message … We know Al Qaeda is out there and a danger to us all. He's threatening the whole world. The fact is there's still a threat out there and we all need to continue to cooperate fighting terror."

Boucher said the State Department heard a lot about the tape in advance of its airing and that it supports the White House's assertion that there are links between bin Laden's Al Qaeda and Iraq.

"It's clear there is a link; it's clear there's communication, there's channels, there's contact," Boucher said. "We know of eight high-level meetings."

Al-Jazeera's chief editor, Ibrahim Hilal, said the 16-minute tape was a message to Iraqis to remain steadfast in the face of a potential American attack.

Yasser Thabet, a broadcast editor, said the tape appears to be authentic because the television station got it through the same means as previous bin Laden statements. He did not elaborate.

"When you listen to the tape, you can tell it's Usama bin Laden's voice," Thabet said.

The last bin Laden tape aired on Nov. 12 on Al-Jazeera. Bin Laden, in the statement, promised new terrorist attacks.

The tape is being analyzed at the National Security Agency and being compared to other tapes "known to be bin Laden," Fox News has learned. It will be run through special computer audio equipment to measure tone and inflection.

U.S. officials said, in this current threat environment, "This timing of this tape is something to be concerned about."

Previous tapes have triggered Al Qaeda cells across the world to act. The intelligence community believes this may tie in with all of the recent "chatter" that they have been hearing about planned attacks.

Al-Jazeera satellite channel received the tape Tuesday evening, according to Jehad Ballout, media-relations manager. It is not known when the actual recording of the message took place. The way Al-Jazeera received the latest audio recording was similar to the way the Arab language news channel received previous audio and visual recordings by bin Laden.

Fox News' Jim Angle, Teri Schultz and Bret Baier and The Associated Press contributed to this report.