By Jonathan Marcus BBC Defence Correspondent

The Royal Marines are Britain's sea-borne fast response unit - elite light infantry units that can be deployed at very short notice and operate in all kinds of difficult terrain.

The need for mobility means that they do not have much of the heavy weaponry that, for example, the US Marine Corps can deploy.

But their mission in Afghanistan is typical light infantry work depending as much upon the level of training of the individual marine and his unit as it does upon firepower.

Some of the Royal Marines are said to be operating above the snow line with night-time temperatures well below freezing.

The terrain and weather in Afghanistan present as many problems as the enemy and in these conditions neither side can deploy heavy equipment.

Attack helicopters

The Royal Marines are depending upon their individual rifles backed up by heavier support weapons like mortars and anti-tank missile launchers.

What heavy fire-power they have is provided by a battery of 105mm light guns which are highly accurate and highly mobile.

They can be slung under the Chinook helicopters which are supporting the Royal Marines' deployment.

The Royal Marines from 45 Commando are working as part of the wider US-led operation and can clearly call upon US fire-power, including attack helicopters or ground attack aircraft if required.

It is not clear though if the current operation - dubbed Operation Ptarmigan by the Ministry of Defence - involves any US troops.

Superior training

Ministry of Defence spokesmen have released few details of the marines' current mission.

It is said that they are combing an area in which al-Qaeda and Taleban elements have been operating.

But it is not clear if they are likely to encounter armed resistance, or whether they are simply destroying caves, ammunition supplies and so on.

The average Afghan fighter is similarly lightly armed using Kalashnikov rifles and light support weapons.

But the standard Soviet-style RPG or rocket propelled grenade is a highly effective weapon in these sort of circumstances and the marines will be relying upon superior training, accurate intelligence, and their own tactical skills to outwit a highly capable enemy.