Afghan fighters found no sign of Bin Laden in the caves
US troops apparently intercepted radio messages last week which showed that Osama Bin Laden was still commanding his forces in the area.
The intercepts, reported by unnamed US officials to the media, have not been officially confirmed but a prisoner said Bin Laden had not been there in over a week.
Other media have been running reports that Bin Laden left Afghanistan long ago and the commander of the US forces in Afghanistan, General Tommy Franks, has admitted that nobody really knows his whereabouts.
A Yemeni fighter captured at Tora Bora said the al-Qaeda leader was last there 10 days previously.
Unnamed US officials said Bin Laden's voice had been identified by comparing it to several videotapes featuring the al-Qaeda leader.
The recording could have been made by specially equipped troops on the ground or instruments on aircraft and satellites.
An Afghan commander fighting alongside the US forces in eastern Afghanistan, Atiqullah Rachan, said that he was unaware of any intercepts although the Pentagon, he added, had "their way of knowing things".
US Defence Secretary Donald Rumsfeld said that he had no reason to believe Bin Laden would not be found.
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"I'm going to wait and see where he is," he said.
"I've received nothing that is discouraging. We continue to receive mixed messages."
Last contact
The last confirmed sighting of Bin Laden appears to be his appearance in the video found by US troops where he describes his role in the 11 September terror attacks on America.
That tape is believed to have been made in Afghanistan in November.
Media in Pakistan and elsewhere have been carrying reports that Bin Laden escaped from Tora Bora long ago.
The last journalist believed to have interviewed Bin Laden, Hamid Mir of the Pakistani newspaper Ausaf, has told the French news agency AFP that he does not think the fugitive is in Tora Bora.
"I don't know whether he is in Pakistan or in the southern part of Afghanistan," said Mr Mir.
An earlier report from the Pakistani newspaper al-Akhbar said Bin Laden and the Taleban leader Mullah Mohammad Omar had been shot with their own consent in Kandahar the day the city fell to anti-Taleban forces.
Outside the region, French TV has reported the possibility that he left Afghanistan by helicopter, for either Yemen or Africa.
A BBC regional analyst says that Bin Laden probably has no intention of being taken alive and put on trial.
"He seems ready to die, in the firm belief that after him will come other Bin Ladens to continue the struggle," he says.
Outraged by the al-Qaeda leader's filmed remarks about 11 September, Washington is stepping up its hunt.
They are offering $25m for information from local people - or possibly captured Taleban fighters - on where Bin Laden could be hiding.
Radio broadcasts and leaflets dropped from US planes are telling Afghans about the reward for Bin Laden's capture.