Adam J. Szubin, the acting junior secretary for terrorism and financial intelligence at the US Treasury Department has painted a grim picture when discussing the financial health of the so-called 'Islamic State.'

"We've seen them significantly increase taxation rates and increase the categories of activity they're now taxing ... While they had once held off from taxing the poorest civilians on humanitarian grounds, they are now taxing across the board," he told the CTC Sentinel - a counter-terrorism publication.

Mr Szubin says that overall its financial outlook has become, "significantly constrained."

The US official added that death benefits have been cut, "That's a core benefit that a group like ISIL needs to promise to the families of those going on suicide or likely suicide missions in order to maintain their operational tempo," he continued.

It has been widely reported that its fighters have taken pay cuts, although Mr Szubin says that he believes that top ranking individuals involved in the armed struggle are unlikely to have been asked to take lower wages. He believes the bulk of these cuts were aimed at foreign fighters.

Over the past six months IS has lost territory and felt the strain of falling oil prices - "If you look at a number of the military fronts, they're feeling strain there. I think they're feeling it in terms of the public narrative. And they're feeling strain in terms of the internal support by Sunni populations who have soured even further on ISIL. They are back on their heels," Mr Szubin concluded.