According to President George W. Bush, the US-led campaign against terrorism will include "covert operations secret even in success". The phrase suggested key elements will be intelligence and special operations, backed up by massive air and seapower being assembled by the US and Britain in the Indian Ocean region.

The stage is therefore set for extensive use of "special forces", elite troops trained in exceptional skills for unconventional warfare. According to reports that defence officials will not confirm, their infiltration into Afghanistan has already begun in an attempt to track down Osama bin Laden, chief suspect in the attacks.

Although many units of the US and UK armed forces can carry out special operations, only a few really have the right to be called special forces. In the US, they include the counter-terrorist Delta Force and Navy Seals, a reconnaissance force in the Marine Corps. The US Army has special forces groups known as Green Berets, as well as Rangers with special operations capabilities. In Britain, special forces include the army's Special Air Service, the Royal Marines' Special Boat Service and units such as 14 Company which specialises in intelligence.

True special forces go through extremely rigorous training; they can go behind enemy lines for long periods and although bonding with team mates is important, they are taught to act and survive as individuals. They are exceptionally tough men although they are not known as wild. Brain is as important as brawn and weaponry.

Andy McNab, author of Bravo Two Zero, an account of an SAS patrol in Iraq, wrote of the training course: "Some of the people who turn up are complete nuggets. They think the SAS is all James Bond and storming embassies. They don't quite understand that you are still a soldier."

Terry Griswold and D.M. Giangreco wrote in a book on Delta Force: "The last thing a unit needs in the midst of a mission is to find itself saddled with a would-be Rambo or a real-life psycho."

Volunteers for the SAS undergo a six-month course including intense physical exercise, weapons skills, navigation, escape and evasion, survival, jungle warfare, close proximity combat and living off the land. Trainees are repeatedly deprived of sleep. One former special forces officer said he was one of three men who completed it out of a hundred who started.

Mr McNab wrote: "You're turned loose on the Black Mountains dressed in second world war battledress trousers and shirt, a greatcoat with no buttons, and boots with no laces." Then you are chased by hunters with helicopters. When they catch you, you are grilled. The former officer says: "You are subjected to intensive interrogation - stripped naked, hung up by your ankles, abused, the whole thing."

US Navy Seals training includes "drownproofing": 30 minutes in water with ankles bound together and hands tied behind backs. The course's "Hell Week" culminates in crossing the "demolition pit" on ropes strung above murky water.

Two contrasting events in 1980 set the tone for recent special forces' roles: the successful storming of the occupied Iranian embassy in London by the SAS; and the failed attempt by Delta Force - then recently formed and modelled on the SAS - and other units to rescue Americans held in the US embassy in Tehran. These missions revived governments' recognition of the value of special forces.

The main opponent of UK special units has been Irish terrorism. SAS soldiers were sent - as was Delta Force - into Iraq to locate Scud missile launchers in the Gulf war. Special forces have been used in the Balkans, central and south America, Africa and even in Afghanistan during the Soviet occupation. Last year, an SAS soldier was the only Briton to die in the rescue of troops held in the Sierra Leone jungle. There have been many direct actions against terrorists. Most missions never become public.

In Afghanistan, special forces may perform a number of roles. Given adequate intelligence on likely targets, they can enter on foot or by specialised aircraft and get as close to the site as possible to carry out reconnaissance. They are skilled in travelling and watching undetected - they can stay still for days. Mountaineering ability is less important than survival skills. They can direct precision-guided munitions onto a terrorist hide-out or Taliban military base. The forces will include snatch teams and raiding parties.

Special forces have exotic weaponry and can call on formidable help, such as Spectre gunships and Pave Low helicopters, as well as intelligence from satellites and spy aeroplanes, an increasing number of which are unmanned. But the resources that make them uniquely useful are those acquired in training. "Ultimately, it comes down to a guy with a gun within shooting range or a knife within cutting range, and that's incredibly difficult," the British former officer says.

The allies' military build-up

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US Air Force B-52 bombers capable of dropping or firing long-range cruise missiles and an array of other weapons have been deployed to an undisclosed location. The Air Force is sending an additional 100 to 130 aircraft to the Gulf region, including B-1 "stealth" bombers and fighters, to join some 350 aircraft already in the region.

* 10,303 members of the US National Guard Air Force Reserve troops have been called up so far, including units to provide air refueling and joint communications.

* Lt Gen Charles Wald, commander of US Central Command's air component, has moved operations to a command and control centre at Prince Sultan Air Base in Saudi Arabia.

* In Japan, the USS Kitty Hawk, the only US aircraft carrier stationed in the western Pacific, left its port in Yokosuka for an undisclosed location. Two carriers, the USS Enterprise and Carl Vinson, are already in the Gulf or Indian Ocean and a third, the Theodore Roosevelt, left Norfolk, Virgnia, during the week for the Mediterranean Sea and possibly points east.

* The British Armed Forces is currently conducting a training exercise of 20,000 troops in the Gulf region, planned before the US attacks. A group of seven Tornado aeroplanes left RAF Marham in Norfolk on Sunday for Oman to join the exercise. The naval task force is led by UK's carrier, HMS Illustrious.