The men had been accused of spying on ISIS on behalf of Iraqi government security forces, according to The Daily Mail.

According to witnesses, the 25 alleged 'spies' had been tied together with a rope and lowered in a large basin containing nitric acid until their organs dissolved.

Nitric acid is a colorless, yellow or red, fuming liquid with an acrid, suffocating odor which is highly corrosive to all parts of the human body.

Read alsoISIS declares state of emergency in Raqqa – U.S. officialIt is normally used in manufacturing ammonium nitrate for fertilizer and explosives, organic synthesis, photoengraving, etching steel, and reprocessing spent nuclear fuel.

The executions in Mosul follows a number of deadly bombings in the capital Baghdad, as ISIS hopes to rebound from a series of battlefield losses in Iraq.