The so-called Islamic State has lost 26 percent of the income it had been generating from the sale of crude oil, according to new research released on Monday.
The report, by conflict monitoring group IHS, showed that production of oil in ISIS-operated oilfields has dropped to approximately 21,000 barrels per day, from 33,000 during the summer period last year.
Almost all the main oilfields operated by the group have been targeted by U.S. military airstrikes, reflecting the intensification of the U.S.-led coalition against the group in Syria and Iraq.
Overall, ISIS revenues have dropped 30 percent in the last year.
IHS senior analyst Ludovico Carlino said, "In mid-2015, the Islamic State's overall monthly revenue was around $80 million. As of March 2016, the Islamic State's monthly revenue dropped to $56 million."
Approximately half of the group's revenue comes from taxation and confiscation; however, this has fallen too due to loss of territory and population under ISIS' control dropping from 9 million to 6 million people.
"The Islamic State has lost about 22 percent of its territory in the past 15 months," said Columb Strack, senior analyst at IHS. "There are fewer people and business activities to tax; the same applies to properties and land to confiscate."
ISIS has increased taxes on basic services including adding tolls for truck drivers, fees for anyone installing new or repairing broken satellite dishes, and "exit fees" for anyone trying to leave a city, reported IHS.
To make up for lost revenue, the Islamic State is also imposing fines. "You can be fined for driving on the wrong side of the road and for not being able to answer questions correctly on the Quran," Carlino said.
ISIS has started to accept payment of fines in cash as an alternative penalty to corporal punishments imposed by Sharia law, according to IHS.
In January, leaked internal documents from the terror group, obtained and translated by Aymenn Jawad Al-Tamimi, showed that ISIS had allegedly cut its fighters' salaries by as much as 50 percent.