A SAS sniper has beheaded an ISIS executioner with a single shot while the militant was teaching jihadis how to decapitate prisoners, it has been reported.

Some 20 Islamic fighters were taking part in the executioner's outdoor lesson when he was killed by the elite British soldier - hiding 4,000ft away and using specially-designed 'wounding' bullets.

A witness said the entire group of student jihadis then fled, deserting their cause after the ill fated training session at a small village in northern Syria.

The SAS sniper reportedly killed the ISIS commander while he was training 20 other jihadis (file photo)

A witness told The Daily Express the SAS sniper was using a Dan.338 rifle with a suppressor to prevent any noise and flash giving away his position.

Aiming a foot to the side of his target in order to adjust for wind, it was expected the cruel commander - known as the local executioner - would require two shots to kill.

Instead, the first bullet - designed to tumble when it strikes a target, in order to maim - struck him in the back of the head.

The source said: 'One minute he was standing there and the next his head had exploded.

'The commander remained standing upright for a couple of seconds before collapsing and that’s when panic set in.

'He was an extremely sadistic and ruthless individual, feared by the locals and the jihadis alike.'

British Prime Minister David Cameron has refused to engage in a ground war with ISIS troops in Iraq and Syria, but small SAS teams are carrying out commando raids against the jihadis.

Earlier this month it emerged three British Special Forces soldiers were wounded in a firefight with 30 ISIS militants in Iraq.

The SAS and SBS were said to have been on a secret mission deep inside enemy territory when the terrorists opened fire on them with machine guns mounted on American-built Humvees.

The men's bravery has not been acknowledged because defence chiefs refuse to confirm the ground war being fought by elite British troops in war ravaged Iraq and Syria, it is claimed.

The ISIS commander was teaching 20 men how to decapitate when his own head 'exploded' from the specially-designed wounding bullet used in the SAS shot

It is thought the men injured by bomb blasts during a 25 man patrol in northern Iraq were the first British casualties in the conflict, the Mirror reported.

German, American and British soldiers were said to be hunting down ISIS fighters outside the terror group's stronghold of Mosul when the terrorists opened fire with 50 calibre guns and rockets.

The elite troops dove from their vehicles and returned fire with their high powered assault rifles.

A unit of German commandos is said to have destroyed one ISIS location with an armour piercing missile - killing every extremist inside. All of the fanatics were said to be killed in the 15 minute battle and an airstrike that followed.