ISIS fighters left out of pocket by bombing, slide in oil

Daesh followers were left out of pocket towards the end of 2015 after the terrorist group cut their salaries by up to half, according to leaked documents.

No one would be exempt from the reductions, although foreign fighters were expected to continue receiving better treatment.

"So on account of the exceptional circumstances the Islamic State is facing, it has been decided to reduce the salaries that are paid to all mujahideen by half, and it is not allowed for anyone to be exempted from this decision, whatever his position. Let it be known that work will continue to distribute provisions twice every month as usual," reads the document, which was translated by Aymenn Jawad Al-Tamimi, a research fellow at the Middle East Forum, CNBC reported.

The typical militant for the group could expect to earn between $400 and $1,200 a month, on top of a further $50 for their wife and another $25 per child.

Nonetheless, according to the Syrian Observatory for Human Rights the reductions were directed mainly at those furthest from the frontlines, at Daesh's capital in the captured city of Al-Raqqa.

As a result of the cuts to the handouts, the movement's fighters stood to lose approximately $200 a month, the observatory said.

Oil's fall to 12-year lows and the aerial bombing campaign against Daesh's oil production and distribution assets were thought to have added to its financial straits.

Daesh had budgeted $2bn for 2015, including a $250m surplus, CNBC added.