Osama Bin Laden is said to be preparing to release a video message to the American people to coincide with the sixth anniversary of the 9/11 attacks.
The announcement was made on an Islamist website, where al-Qaeda's media arm frequently posts messages.
US homeland security officials could not confirm the existence of a tape, and said there was "no credible information of an imminent threat".
Al-Qaeda has put out similar statements in the past but no video has followed.
Bin Laden has not been seen in a video since October 2004, when he threatened new attacks against the US on the eve of the presidential election.
An audio tape was released in January 2006.
Dyed beard?
A banner advertisement for the video, written in Arabic, read: "Soon, God willing, a video tape from the lion sheikh Osama Bin Laden, God preserve him."
It carried an al-Sahab logo - regarded as a signature of al-Qaeda recordings.
There were no indications of the content of the video or when exactly it might appear.
The website also included a photograph, which appeared to show Bin Laden has aged compared with previous pictures - although his beard now looks dark rather than greying.
Correspondents say a new video - if indeed one does appear - would serve to dispel persistent rumours that the al-Qaeda leader might have died.
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