US Senator Bob Graham, head of the Senate's Intelligence Committee, is convinced Osama Bin Laden is still alive.

The senator, who is regularly briefed by US intelligence agencies, told Panorama's Jane Corbin: "Our intelligence sources say that they think he's (Bin Laden) still alive and they believe he probably is in those tribal territories between Afghanistan and Pakistan.

"Can they give you an address, zip code and telephone number? No.

"There is compelling evidence that in fact a significant part of what remains of the hierarchy of al-Qaeda has moved from Afghanistan into Pakistan and would try to regroup and regenerate from there."

Threat from Pakistan

Senator Graham adds: "The worst case scenario would be if a terrorist group such as al-Qaeda were to gain access to the arsenal of nuclear weapons and missiles which the Pakistani Government has and use them for terrorist purposes."

Musharraf: Al-Qaeda in Pakistani cities cannot be ruled out

And General Perez Musharraf has admitted that al-Qaeda has infiltrated Pakistani cities.

Speaking to Panorama, the Pakistani president said: "The possibility of their (al-Qaeda terrorists) moving into our cities is always there and that is what was proved when we moved against their groups in Faslabad and Lahore and got about over 40 of them, and Abu Zabada.

"So therefore I cannot rule out they're being in our cities, but they will be in small numbers, in penny packets, that is a surety."

'Osama was at Tora Bora'

In the programme, Jane Corbin follows the escape route taken by the al-Qaeda leader from the Afghan capital.

During the battle in Tora Bora it was possible for al-Qaeda to escape... there was some kind of dishonesty

Afghan warlord Hazret Ali

Afghan warlord Hazret Ali, who fought alongside the Americans at the Tora Bora battle, tells Panorama: "Osama, as far as I knew, was at the battle at Tora Bora.

"One of the prisoners who was captured told us that he saw him with his own eyes and I also had information from my own sources that Osama was out here at Tora Bora."

And he confirms suspicions that treacherous local warlords, supposedly helping the Americans, enabled Bin Laden and his most hardened fighters to escape from the battle.

"During the battle in Tora Bora it was possible for al-Qaeda to escape - I cannot point to anyone but I think there was some kind of dishonesty."