In a report released on April 18, IHS says revenue for the group fell to $56 million in March from $80 million a month in mid-2015, RFE/RL reports.

It says daily oil production in IS-controlled areas went down to 21,000 barrels from 33,000 barrels over the same time frame as production facilities suffered damage from air strikes.

Read alsoISIS used mustard gas during attack on Syrian military airbase in Deir EzzorMeanwhile, IS militants have lost 22% of their territory in Syria and Iraq and now rule over 6 million instead of 9 million people – meaning the tax base for the group has become smaller.

In consequence, they have introduced new taxes on such activities as installing satellite dishes or exiting cities.