Latest macabre execution in Mosul saw 25 people lowered into vats of nitric acid 'until their organs dissolved.'

As the ISIS terrorist group continues to lose territory - while calling for increased attacks against the West - it is also ramping up its brutal efforts to deter would-be spies, and keep the people it rules in Iraq and Syria entirely under its control.

Infamous for its particularly gruesome, and public, execution methods, the so-called "Islamic State" reportedly outdid itself in cruelty last week, killing 25 people accused of spying by dissolving them in acid.

The macabre spectacle reportedly unfolded in the group's Iraqi stronghold of Mosul, where Kurdish forces, backed by US air power, are putting the squeeze on the jihadists.

The Iraqi News site first reported on the public execution of the men, who ISIS alleged had acted as spies for Iraqi security forces, who are also advancing on Mosul from the south.

"ISIS terrorist members executed 25 persons in Mosul on charges of spying and collaborating with Iraqi security forces," a witness told the site. "ISIS put the citizens in a large tub containing nitric acid inside one of its headquarters."

The jihadists "tied each person with a rope and lowered him in the tub, which contains nitric acid, till the victims organs dissolve[d]," he added.

Nitric acid is a highly corrosive and poisonous liquid, most commonly used to manufacture ammonium nitrate for fertilizer and explosives, as well in the production of steel.

ISIS has lost around 45% of the territory it once held in Iraq, and some 20% of the territory it held at the height of its power in Syria.

Experts have speculated that a recent string of suicide bombings in Iraq, as well as recent terrorist atrocities in Europe and threats of further attacks on US soil, are the group's way of attempting to signal to its supporters that it is still relevant, despite have lost its momentum on the battlefield.