The ISIS terror group has seen its revenue fall by 30 percent from last year while the number of people living in territory under its control has dropped by a third, a new report says.

The consulting firm IHS reports that as of March 2016, ISIS' monthly revenue was $56 million, down from a high of $80 million in March 2015.

The decrease is due in part to diminished oil production in ISIS-controlled territories. IHS estimates that around 21,000 barrels of oil are produced each day in the self-proclaimed caliphate, down from around 33,000 barrels per day. The report gives airstrikes by the U.S.-led coalition and Russia partial credit for impacting ISIS' oil production, but also notes that the militants were able to repair damaged infrastructure quickly.

The report says that setbacks on the battlefield have cost ISIS some of its main source of income: a tax base that numbered nine million people at the height of the militants' territorial conquests.

"[ISIS] has lost about 22 percent of its territory in the past 15 months,” IHS senior analyst Columb Strack told The Telegraph. "Its population has declined from around nine million to around six million. There are fewer people and business activities to tax; the same applies to properties and land to confiscate."

In an effort to shore up declining revenues, ISIS enforces have introduced more creative penalties for minor offenses. 

"You can be fined for driving on the wrong side of the road and for not being able to answer questions correctly on the Koran," senior analyst Ludovico Carlino told The Telegraph.

The IHS report also claims that ISIS has begun charging "exit fees" for people who wish to leave cities under its control and even accepting cash in lieu of doling out corporal punishment.

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