Downing Street has confirmed that London and Washington are talking about the use of British ground troops in the war in Afghanistan.

No decisions have yet been announced, and army units remain on the same state of readiness as they have in recent weeks.

But when the announcement does come - as seems inevitable - what British troops would be suitable for operations inside Afghanistan?

SAS involvement

Rumours that a handful of men from the Special Air Service (SAS)are already on the ground have been circulating for weeks.

The cream of British special forces, the SAS have a long history of involvement in Afghanistan, training mujahedin fighters in the 1980s.

Their knowledge of the terrain, tactics and personalities involved will make them invaluable to British and American planners.

With the bold motto "Who Dares Wins" and a cloak of official secrecy (government officials routinely answer that "we never comment on special forces" when quizzed about its activities), the SAS enjoys an almost mythical status.

A special unit of the SAS has been involved in mountain training in Pakistan in recent years.

The unit has high altitude experience and is thought to enjoy good relations with Pakistan's own special forces.

Eschewing traditional army organisation, SAS soldiers work in small teams, often spending weeks or even months hidden behind enemy lines.

Their exceptionally rigorous training makes them ideally suited for the difficult terrain and harsh conditions of Afghanistan through the winter months.

There they will gather crucial intelligence, using sniper scopes and night vision goggles to monitor the movement of Taleban and al-Qaida fighters.

Using the latest communications technology, their information will be fired, in short, encoded bursts, up to reconnaissance planes flying overhead.

Royal Marines

Despite its elevated reputation, it is not thought likely that more than 100 members of the SAS could be deployed in Afghanistan.

However, some reports suggest that the Pentagon is anxious to see the whole regiment deployed. This would involve waiving the traditional stipulation that no more than two of the four "Sabre" squadrons can be deployed abroad at any one time.

3 Commando Brigade is also likely to figure prominently in any ground operations.

Already described as "suitable" by the Chief of the Defence Staff, Admiral Sir Michael Boyce, regular arctic training in Norway makes them better equipped than most to deal with the Afghan winter.

A core component of the UK's Joint Rapid Reaction Force, it is on permanent readiness to deploy across the globe.

With elements of the SAS, the brigade is currently involved in Exercise Saif Sareea ("Swift Sword") in Oman. Planned for more than three years, the exercise places British armed forces, including substantial naval and air force assets, conveniently close to the region.

Said Sareea will start to wind down later this month, but 3 Commando will remain in the area until mid-November as part of an amphibious training exercise in the Arabian Sea.

The brigade is held in high regard, having successfully fought the six week campaign to win back the Falkland Islands from Argentine forces in 1982.

Since then, involvement in protecting Kurdish refugees in 1991, peace-keeping duties in Bosnia-Herzegovina and Kosovo have kept the brigade busy.

Afghanistan may be landlocked, but the Marines' own elite unit, the Special Boat Service, who often train and exercise alongside the SAS, could also be deployed.

Another tiny unit, the Brigade Patrol Troop, whose 24 men work in six four-man teams, are specialists in collecting information on the enemy and topology.

The Parachute Regiment

Vying with the Royal Marines for the job of supporting special forces, the Paras form the backbone of 16 Air Assault Brigade, a formation created to punch quickly into enemy territory.

The Commandos may have the advantage of cold weather, mountain training and current proximity to Afghanistan, but the Parachute Regiment is understood to be lobbying hard, emphasising its skill in helicopter assault tactics and its recent contribution to the successful collection of Albanian rebel weapons in Macedonia.

The specialist Pathfinder Platoon could be covertly deployed as much as a week before the arrival of a main force. Pathfinders then conduct a range of intelligence roles, including target acquisition and reconnaissance, usually behind or close to enemy lines.

According to 16 Air Assault's website, Pathfinders "specialize in air insertion, most notably through the parachute techniques" from fixed wing aircraft and helicopters. "All personnel are also fully conversant, and regularly practiced in Special Forces procedures."