Osama bin Laden is fast losing his authority over his al Qaida network, the US claimed last night as anti-Taliban fighters closed in on his suspected mountain hideout.

US Deputy Defence Secretary Paul Wolfowitz said there was no "credible evidence" to suggest that bin Laden was anywhere other than Afghanistan's Tora Bora mountains.

He told a Pentagon news conference that the al Qaida leader's ability to communicate with his supporters outside Afghanistan appeared to have been "substantially degraded" as a result of the military action against him.

"I think we have probably substantially reduced his authority over people who might be inclined to listen to him," Mr Wolfowitz said.

"This is a man on the run; a man with a big price on his head; a man who has to wake up every day and decide 'Do I keep all this security around which I need to make sure that some Afghan bounty hunters don't turn me in but which help to give a lot of reports about my whereabouts, or do I go into hiding?'

"He doesn't have a lot of good options."

After a day of fierce fighting, Afghan forces loyal to the Eastern Shura a local anti-Taliban force were reported to have captured a key ridge close to the formidable cave and tunnel complex in the Tora Bora mountains.

Hundreds of Eastern Shura fighters mounted a three-pronged assault towards the Tora Bora caves, supported by US warplanes and the fire of their ageing T-55 Soviet-built tanks.