A combination of successful airstrikes by Western governments and drop in the oil price are helping to cut off a much-needed revenue stream for the so-called Islamic State (ISIS) terrorist group, an energy analyst says.
"The damage is being done, which is why we have seen ISIS oil production go from 110,000 barrels a day to 40,000 barrels a day," Amrita Sen, chief oil analyst at Energy Aspects, told CNBC Thursday.
Following Friday's terrorist attacks in Paris and confirmation that a bomb brought down a Russian airliner in the Sinai desert a couple of weeks ago -- attacks both pinned on ISIS -- the U.S., France and Russia have stepped up airstrikes on the militant group's positions in Iraq and Syria.
The key targets for airstrikes have been ISIS' main sources of revenue and operations, including oilfields, trucks used to transport oil, munitions depots, command centers and trade routes (used mainly to transport arms and oil). Earlier this week, the U.S. said it had destroyed over 100 fuel trucks in an airstrike.
ISIS controls more than 60 percent of Syria's oil production capacity and about 10 percent of Iraq's oil production capacity, Reuters estimates. In an October 2014 report, it estimated that the group had an overall production capacity of up to 120,000 barrels per day, representing a profit of $2 million to $4 million per day.
Since last year, however, the price of oil on global markets tumbled from a high of $114 a barrel last June to around $44 for a barrel of benchmark Brent crude and around $40 for U.S. crude, further impacting one of ISIS' main revenue streams.
"At the peak, ISIS had about 100,000-110,000 barrels a day under control but this was back in the summer of 2014. Now, and with particularly all the airstrikes, it's less than 50,000 barrels per day – we think that it's about 40,000 per day."
"Some of that they probably have to use internally for their own consumption and they're probably exporting some but again, with oil trading around $40 a barrel, they're probably not getting as much revenue as they were last summer."
There have been reports that ISIS has sold its oil on the black market, and even back to the Syrian regime led by Bashar Assad that it is fighting in Syria's civil war. There have also been reports that ISIS' oil has been smuggled into Turkey, a country also opposed to ISIS.
Sen believed that it was plausible that Syria had received ISIS'-sourced oil and possible that Turkey had received oil from ISIS. "Some oil was smuggled into Turkey -- not necessarily that Turkey's buying it – but it just gets mingled with other oil being imported there."
- By CNBC's Holly Ellyatt, follow her on Twitter @HollyEllyatt.