WARNING: Graphic content.

“ONE minute he was standing there and the next his head had exploded.”

That’s how one witness described the shocking moment a British SAS sniper hit his target — an Islamic State executioner.

Ironically, the victim was teaching a group of young recruits the dark art of beheading when he was decapitated by a single bullet from a long range rifle, reportedly from a distance of 1,200 metres.

The slug in question is known as a “wounding” bullet and makes a “tumbling motion” when fired. That means it basically cartwheels around a body instead flying straight through flesh — with devastating effect.

The Daily Expressquoted an unnamed source who identified the sniper’s weapon as a Dan .338 equipped with a suppressor “which reduces the sound and eliminates any flash from the barrel”.

“He had to aim off by more than a foot,” the source told the paper. “One minute he was standing there and the next his head had exploded.”

The incident reportedly took place at a remote desert location in northern Syria a fortnight ago following a tip off by British spy agency MI6.

Operatives had received information that ISIS tactical training was being conducted in a small village near a school and that young recruits were being taught how to behead people using knives, axes and swords, The Mirror reported.

The area was reportedly too inaccessible for air strikes to be effective and so an elite British team of eight, armed with long range sniper rifles, machine guns and rocket launchers was sent in to disband the camp. Another 12 SAS troops waited nearby in heavily armoured military vehicles, the Mirror said.

It is not clear whether the action was in retaliation for last month’s execution of five British “spies” as an apparent “direct threat” to British PM David Cameron.

The identity of the executioner killed by the sniper has not been released but it is unlikely to have been Englishman Siddartha Dhar, who is rumoured to have replaced Jihadi John after the fellow Brit was killed in a drone strike late last year.

Last week news.com.au reported that a “mystery sniper” had killed three ISIS chiefs in Sirte, the terror group’s Libyan caliphate, over a 10-day period.

The identity of the gunman and for whom he or she works has not been uncovered but locals refer to them as “The D’aesh Hunter”.

The sniper’s victims have included Hamad Abdel Hady, a Sudanese national working for the newly-established Sharia court, was taken down by a sniper’s bullet outside a hospital in mid-January, according to The Libya Prospect newspaper.

Next to die was ISIS leader Abu Mohammed Dernawi, who was killed on January 19.

Then Abdullah Hamad Al-Ansari, an ISIS commander from the southern Libyan city of Obari, was shot leaving a mosque on January 23.

Rumours continue to grow that a shooter is systematically targeting IS commanders one-by-one, according to UK newspaper The Telegraph.