US bombing on ISIS oil facilities is finally taking its toll, hindering the Islamic State's ability to earn from this business, which is fueling most of its terrorist activities.

Time reported that oil sales is one of the most important sources of income for ISIS. The group sells up to 40,000 barrels a day, which translates to roughly $1 million every day. The ISIS caliphate earns up to $500 million a year from its oil business.

Defense Secretary Ashton Carter said that the US-led alliance has stepped up its attack against ISIS oil facilities in October. Before that month, they have been bombarding 'largely small-scale" ISIS oil operations for the past 15 months.

In a report by CBS News, Bloomberg's Matthew Phillips said, "We overestimated really the damage that we did -- the long-term damage -- and underestimated their ability to kind of adapt to that... on last year's strikes."

U.S. officials now report that ISIS oil production is dwindling. A senior State Department official reported on December 4 that the ISIS oil production operation is receding from "20^th-to-21^st century operation, to a 17^th century operation" due to the attacks carried out by US and its allies.

According to CNN, ISIS is making $40 million a month from its oil business. Now, with the constant bombing campaign, the terrorist group is earning only a fraction of that amount. American officials, however, say that it is too early to know exactly how much damage has been done against the Islamic State.

"We're already seeing a very significant slowdown in the operation," the senior State Department official said at a recent press briefing. "There is still a functioning capability, but without a doubt, it's under strain."

After the Paris attack on November 13, the US-led attacks have intensified, blowing up 283 oil tanker trucks, 120 storage tanks, an a very big amount of oil field equipment located in eastern Syria.