American intelligence sources told ABC News that the video message was authentic, recently produced and proves that the al-Qai'da leader is still alive. US authorities have a transcript, which they say encourages potential suicide bombers to commit missions against the West.

Evidence that Bin Laden is still directing al-Qai'da operations, despite the best efforts of the US military and a $50m (£25m) price on his head, will be a blow to President George Bush who staked his reputation on winning the "war on terror".

In a banner ad for the video, the al-Qai'da leader, now aged 50, looked fit, with a full beard of dark black hair, rather than his usual grey-streaked beard. Instead of the customary combat jacket, he was wearing Arabic robes.

"It does look oddly as if he is wearing a false beard," Richard Clarke, a former White House counter-terrorism official, said. "If we go back to the tape three years ago, he had a very white beard. This looks like a phoney beard that has been passed on."

The al-Qai'da leader has not appeared in a video message in almost three years and has not put out a new audio message for more than a year. His last video broadcast was days before the 2004 presidential vote.

The new videotape is expected to be released in the next few days or hours and may also be timed to coincide with next week's debate in Congress about the unpopular war in Iraq and demands for a pullout of forces.

US intelligence analysts are trying to determine the tape's authenticity and how recently it has been recorded. A government official said, on condition of anon-ymity: "We need to take a look at it to see whether it's old or new, and we're doing it very quickly."

Al-Qai'da's propaganda arm, Al-Sahab, announced the imminent release of the video on Thursday on an Islamic website often used by the terrorist group. "Soon, God willing, video message by the Lion Sheikh Osama bin Laden, may God protect him," read a banner ad in Arabic. US intelligence will examine whether Bin Laden mentions current events, to help establish when and where the tape was produced. In his last taped message, Bin Laden said he decided in 1982 to attack the World Trade Centre after Israel invaded Lebanon, backed, he said by the US Navy.

The US Department of Homeland Security said there was no "credible information" about any imminent threat to the US, but repeated the department's message that the US is "in a period of increased risk". The National Security adviser Fran Townsend, told CNN a video from al-Qa'ida would just be propaganda. "People have got to be clear that we're being manipulated every time they issue a statement, because they're trying to use the media as a way to terrorise us," she said.

US officials say Bin Laden may be hiding in tribal areas in the mountains on the Afghanistan/Pakistan border. There has been no good lead on the terrorist leader's hideout since 2005.