The deployment to Afghanistan will be one of the stiffest assignments ever handed to 45 Commando, the mountain warfare marines who left Scotland to hunt down al-Qaeda and Taliban guerrillas.

"The young guys are very excited. The older guys are more cautious," Major Rich Hills said yesterday against the crackle of automatic fire at his unit's Arbroath firing range. "I wouldn't use the word 'worrying'. But I would use the word 'thought-provoking'."

Within the next few weeks, 400 of the unit's marines in Arbroath will join the rest of the corps' 250 men, currently in the Gulf aboard HMS Ocean, in Britain's biggest deployment of fighting troops since the Gulf war in 1991.

For most of the men, whose average age is 21, it will be their first taste of "war fighting operations", as defence secretary Geoff Hoon called their mission.

45 Commando believes itself to be the "force of choice" for the task of tracking down the remnants of al-Qaeda and the Taliban. "The men have been straining at the leash," said Major Richard King, who will oversee the deployment.

The unit, created in 1943, believes itself to be the best-trained, fittest and most deadly mountain-fighting unit in Nato.

Moreover, its commanders are neither daunted by the Soviet Red Army's bloody failures in Afghanistan, nor by the apparently modest recent success of the far more heavily armed and technologically superior US forces.

Unlike their US counterparts, they say, 45 Commando specialises in despatching nimble units of 60-100 men at a time into hostile mountain terrain without the same need for vehicles or other back-up.

45 Commando will deploy in the mountains of south-eastern Afghanistan, largely on foot. Each marine deployed will carry only 100lbs of guns, ammunition, food, water and bivouac equipment.

But for all their prof-essional self-belief, there was no hiding a sense of trepidation at 45 Commando's base yesterday. "We all know that there may be some of us who won't come back," said one marine who preferred not to be named.