British Special Forces have killed at least 20 Taliban fighters in Afghanistan – despite the official line that our forces are merely supporting the Afghan army in an 'advisory role'.

The SAS are involved in close-quarter battles to stop the extremists capturing the strategically important town of Sangin in Helmand Province, where more than 100 UK personnel have previously lost their lives.

It is understood that this is the first fighting in Sangin involving UK personnel since late 2010 when 40 Commando Royal Marines withdrew from the town.

The Mail on Sunday has obtained an exclusive picture showing elite troops posing with Afghan commandos after a fierce battle against the extremists last week.

The Mail on Sunday has obtained an exclusive picture showing elite troops posing with Afghan commandos after a fierce battle against the extremists last week

British Special Forces have killed at least 20 Taliban fighters in Afghanistan – despite the official line that our forces are merely supporting the Afghan army in an 'advisory role'

Our dramatic image from Sangin includes four Special Forces troops armed with high-powered assault rifles and wearing night-vision goggles. It was posted online on Christmas Eve and triggered thousands of supportive messages from Afghans thanking the troops for risking their lives to defeat the Taliban.

Sources close to the SAS confirmed last night that the troops appeared to be British, based on their weapons and clothing, though they stressed that Special Forces from other Nato countries were sometimes similarly equipped and were also involved in secret missions in Helmand Province.

Last night, defence sources described how heavily armed SAS troops joined Afghan soldiers from the 215 Expeditionary Corps to undertake a series of strike operations to try to break the Taliban's siege around Sangin.

For several weeks the insurgents had blocked all major roads leading into the vital town, cutting off supply routes of the poorly equipped Afghan troops and local police holding out here. Some Afghan troops even resorted to posting messages on Facebook pleading for more ammunition and food to be dropped for them.

Sources in Afghanistan said that the SAS used satellite imagery and intercepts of Taliban communications to identify weak points in their defensive lines. Then, on the night of December 22, the elite British troops approached a Taliban checkpoint about 1½ miles north of Sangin on the main road to the neighbouring town of Kajaki.

The SAS guys used their night vision goggles to their advantage and the infra-red target designators which were attached to their rifles

Captured: Taliban prisoners in the custody of the Afghan army last week

A source said: 'It was a classic stealth attack using the cover of darkness. The SAS guys used their night vision goggles to their advantage and the infra-red target designators which were attached to their rifles. This equipment allowed them to identify Taliban gunmen with pinpoint accuracy and eliminate them.

'Officially the British troops were only present in a mentoring role but when they get that close to a fire-fight it is hard to stop them. Publicly speaking their job is to 'train, advise and assist' the Afghans. I think on this occasion the SAS's interpretation of this role was to show the Afghan commandos how the job is done.'

The raid on the Taliban checkpoint on the Kajaki road was one of several that night involving the SAS. British and other Special Forces also accompanied Afghan military units as fighting raged in other districts of Helmand and near the provincial capital, Lashkar Gah.

The Taliban particularly wanted to seize control of Sangin because the town sits on key routes for trafficking drugs and other contraband.

Sangin first came to prominence in 2006 when around 100 UK troops found themselves besieged by the Taliban. For the next four years UK troops fought a losing battle to bring stability to the town until they were eventually replaced by the US Marine Corps.

Members of the Afghan security forces make last minute preparation during an operation in Helmand

The Taliban particularly wanted to seize control of Sangin because the town sits on key routes for trafficking drugs and other contraband

Sangin first came to prominence in 2006 when around 100 UK troops found themselves besieged by the Taliban

The Taliban particularly wanted to seize control of Sangin because the town sits on key routes for trafficking drugs and other contraband

This forlorn yet heroic struggle cost the lives of 106 British soldiers – nearly a quarter of the nation's dead from the 13-year-long combat mission in Afghanistan.

Last night the mother of a British soldier critically wounded in Sangin in 2006 said she was horrified by the thought of troops going back into combat there.

Diane Dernie, whose son, Lance Bombardier Ben Parkinson, suffered brain damage and lost both legs in a bomb blast in the town said: 'It is incredibly saddening. I'm absolutely horrified by the thought we could get drawn back into this unwinnable war. We have already sacrificed so much in Sangin and we haven't learned from our mistakes.'

A senior Nato officer admitted that UK troops are helping their Afghan counterparts 'at a tactical level' – but he insisted that it was the Afghan army and not international troops who were doing the fighting.

US General Wilson Shoffner told The Mail on Sunday: 'In Helmand the British contribution is part of a larger Nato expeditionary advisory effort. But our mission is not combat, the Afghans are doing that and are responsible for the progress on the ground in recent days.

'Helmand continues to be a contested area and we know it's going to be a tough fight. The Taliban's main objective has been to take control of the province. It has failed to do so.'

The revelations about the SAS's frontline role in the battle against the Taliban emerged as British pilots continued their aerial campaign against Islamic State.

In the last three weeks the RAF has completed more than 50 successful air strikes against the insurgents in Iraq – but it has not dropped any bombs on enemy targets in neighbouring Syria since December 6.

Last night, Elizabeth Quintana from the Royal United Services Institute think-tank said the UK was concentrating on supporting the Iraqi Army mission to recapture the key town of Ramadi from IS.