WINNING THE WOT:

The two primary sources of revenue of the group, also known as ISIS -- taxation and oil -- have both dwindled in recent months as Russia and a coalition led by the United States stepped up their airstrikes. According to the IHS report, oil production in areas still under the group's control dropped to 21,000 barrels a day in March from nearly 33,000 barrels a day in mid-2015. And, since mid-2014, the territory under ISIS control shrunk by about 22 percent, thereby reducing its tax base to 6 million people from 9 million.

"Our research has found that the Islamic State [group] is increasing taxes on basic services and coming up with new ways to get money from the population," Carlino reportedly said. "These taxes include tolls for truck drivers, fees for anyone installing new or repairing broken satellite dishes, and 'exit fees' for anyone trying to leave a city."

The militant group has also started imposing monetary fines as an alternative to corporal punishment, IHS said in its report.

According to the group's internal documents leaked in January, in recent months, ISIS has, in order to offset its dwindling source of revenues, been forced to cut its fighters' salaries by as much as half.

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