The brutal fanatics are launching increasingly frequent chemical attacks against Kurdish forces and are even selling young female sex slaves to foreign mercenaries for the same price as a packet of cigarettes, according to Duncan.
The 48-year-old left his self-employed job in Scotland to join Kurdish Peshmerga fighters in Syria and Iraq in a bid to wipe the depraved death cult off the face of the earth.
He claimed terrorist are giving themselves up in droves out of fear of warplane attacks.
But the sniper warned ISIS will become an even greater threat to the UK unless allied airstrikes are supported with equipment and weapons for the brave fighters going toe-to-toe with the evil terror group on the ground.
The Gulf War One veteran told how he witnessed ISIS militants taking drugs on the battlefield during one skirmish.
He said: “It was relentless. ISIS was 2,000 yards away. Then maybe 50 fighters attacked, some as close as 50 yards trying to overrun us but were beaten back.
"ISIS have syringes attached to their clothing so that if they are shot they can give themselves a boost.
“They are high on cocaine and amphetamines. It helps them fight.”
The veteran surveyed a battlefield littered with bodies of dead and dying terrorists before ISIS fanatics dragged them into tunnels and ditches during the night.
He said the deadly attack was just one of dozens his group of fighters witnessed in the region.
Duncan, a former corporal with the Queen’s Own Highlanders, said he felt compelled to assist the Kurdish troops in the fight against savage ISIS.
He said: “I had to act. ISIS is a threat to that area and to the West and only one group is challenging them, the Peshmerga, for whom I have enormous respect.
“When I look at ISIS prisoners I see them as animals because of what they do to people.”
The former corporal, who works unpaid, also detailed how young Yazidi girls in Iraq are sold as sex slaves or exchanged for mercenary work by Chechen gunmen.
He said: “ISIS have sex markets where they buy or sell young sex slaves for as little as a packet of cigarettes or £6 a time.
"Chechen gunmen are bussed to the frontline in exchange for oil to be sold outside Syria or Iraq, or sometimes for sex slaves,” Express reported.
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