'It's almost impossible for outsiders to move without being seen'

Tom Carew is one of few SAS soldiers with experience of fighting in the arid mountains and deserts of Afghanistan, a country where Western soldiers are conspicuous and in permanent danger of discovery.

Twenty years ago, during the height of the Soviet Union's invasion, Carew went to work for MI6 training anti-communist mujahedin forces in the Afghan mountains in guerrilla warfare, operations he described in his book Jihad! The Secret War in Afghanistan. Elite special forces units such as America's Delta Force and the SAS are now, it is believed, involved in similar operations.

Unconfirmed reports that three US soldiers have been captured by the Taliban close to the Iranian border have highlighted the major difficulties that Western forces face: finding trustworthy local allies and keeping their presence as secret as possible. Western military boots are easily spotted, as are blue eyes, high-tech weapons and bulky equipment packs.

"The place is alive with people this time of year," Carew, an adviser to the US Defence Intelligence Agency and Drug Enforcement Administration, told BBC Radio's Today programme yesterday. "They are feeding the sheep on the mountains to fatten them for winter and the place will be crawling with people. You have to be really careful."

The clearest risk is that the Taliban will be searching for foreign soldiers, and will have moved their forces westwards, deeper into Afghanistan. "They will be expecting it," Carew said. "They may have moved most people toward Pakistan; I suspect that's what the special forces are trying to find out. It's one thing having satellite reconnaissance but you need hard intelligence from the ground. And you've got to be very careful at doing that.

"It's virtually impossible to move there in the daytime without being seen, and at night-time, of course, nothing's moving, so you'll get people picking up your movements. They're pretty good at it. They'll see a footprint and they can tell whether the guy's a Westerner. It's by the way he walks." The onset of winter, now four weeks away, increases those risks. Military boots can be easily tracked in snow, and Western special forces – no matter how tough – are quickly outpaced by Afghans.

The SAS and Delta Force, which specialise in counter-terrorism operations, are also trained to track and seize targets using detailed, precise intelligence. They need to know exactly which building or cave Osama bin Laden is using, down to the room he sits in. They also need to establish secure camps and communications with the outside world.

All these factors make the careful construction of relationships and alliances among the anti-Taliban forces absolutely crucial. "It takes time. You've got to get to work with the Afghans and that's not an easy step; you've got to go very slowly until they trust you. It does take weeks to go in. There's no quick thing for this."