ISIS executed 25 alleged spies in Mosul by lowering them into a huge vat of nitric acid, according to several news reports.

A witness to the gruesome killings told IraqiNews.com that each man was tied up with a rope then "lowered in the tub ... until the victim's organs dissolved."

Nitric acid is a highly corrosive mineral acid used in fertilizer and rocket fuel, and also used for etching steel. Even a drop will burn through skin and requires instant treatment.

The brutal executions appear to be a new way ISIS is attempting to keep control of its conquered citizens through fear -- even as its conquered territories shrink through several recent military loses.

Known for beheading and crucifying enemies and alleged spies, the Islamic State appears to be desperately trying to keep its caliphate together in any way possible.

"They are looking for ways to start to regain their momentum or regain the initiative," Army Gen. Joseph Votel, commander of U.S. Central Command, told reporters this week.

Votel said the suicide bombings and other attacks in and around Baghdad over the past week, killing more than 200 civilians, show how rapidly momentum and tactics can change. On Tuesday, suicide bombers struck outdoor markets and a restaurant in Baghdad, killing at least 69 people.