Osama bin Laden said in a new video marking the sixth anniversary of al Qaeda's September 11 attacks the United States was vulnerable despite its military and economic power, but he made no specific threats.
Technical analysis suggests the voice on the video is that of in Laden, a US intelligence official said.
"Initial technical analysis suggests that the voice on the videotape is indeed that of Osama bin Laden," the official said. He said he had no analysis of bin Laden's physical appearance on the tape.
In his first video appearance for almost three years, the al Qaeda leader said US President George W Bush was repeating the mistakes of the former Soviet Union by refusing to acknowledge losses in Iraq.
In a sign that the almost 30-minute tape, acquired by ReutersTelevision from a web trawler in Europe, was made recently, bin Laden mentions new French President Nicolas Sarkozy and British Prime Minister Gordon Brown.
"Despite America being the greatest economic power and possessing the most powerful and modern military arsenal, and despite it spending on this war and its army more than the entire world spends on its armies, and being the major state influencing the world's policies ... 19 young men were able ... to change the direction of its compass," bin Laden said in the tape.
"The subject of the mujahideen has become an inseparable part of the speech of your leader and the effects and signs are not hidden. Since the 11th, many of America's policies have come under the influence of the mujahideen."
The video shows bin Laden sitting at a table dressed in white and cream robes and wearing white headgear. Beneath him, a banner on the screen reads in English: "A message from Sheikh Osama bin Laden to the American people."
Bin Laden appears tired and sallow, though his beard is much shorter and darker than in his last appearance, when it was streaked with grey.
White House spokesman Tony Fratto said the tape demonstrated that "terrorists are out there and they are actively trying to kill Americans and threaten our interests".
Bin Laden was last seen in a video statement issued on the eve of the November 2004 US presidential election. Since then, he has issued several audio messages, the last in July 2006 when he vowed al Qaeda would fight the United States worldwide.
Some intelligence officials and security analysts suggest bin Laden has limited his appearances to maximize their impact, perhaps saving his next one to coincide with a dramatic attack.
Others say bin Laden, aged 50 and believed to suffer from a serious kidney ailment, may be too sick or too tightly pinned down in his hiding place to smuggle out a tape.
US CIA Director Michael Hayden said he had not seen the tape, but he said earlier in a New York speech that al Qaeda had regained strength and its leadership continued to plot a "high-impact" attack on the United States. Asked after the speech if he believed bin Laden was alive, Hayden said: "I have no reason to believe he's not."
A year ago, a leaked French intelligence report said Saudi secret services thought bin Laden had died of typhoid in Pakistan.
US-led forces have been searching for bin Laden since they toppled Afghanistan's Taliban government after it refused to hand over the mastermind of the Septembe 11 attacks.
Bin Laden and his deputy Ayman al-Zawahri are believed to be hiding in the border area between Afghanistan and Pakistan, a mountainous, inaccessible region that US intelligence has described as a safe haven for al Qaeda and the Taliban.