ISIS makes nearly £600million per year from oil revenue, taxes and looting, and have enough cash to fund years of war, an investigation has found.

The militant group spend around two thirds of their annual revenue on its army, reportedly paying foreigners who join their jihad in Iraq and Syria more than twice that of local fighters.

And despite demanding taxes from citizens in areas of occupation and boasting of an 'NHS-style' health care, civilians claim hospitals and public services are falling apart.

Priorities: ISIS makes nearly £600million per year from oil revenue, taxes and looting, two-thirds of which is spent on its jihad army

The investigation, published by the Financial Times, found that of the annual income of £600m ($900m), ISIS spends £400m ($600m) on its army, with £133m-£167m ($200m-$250m) going to its core fighters.

ISIS has enough funds and predicted income keep its military operations running at its current level for another three years, an intelligence official told the FT.

The investigation is based on documents and interviews with former ISIS commanders, civilians, anti ISIS coalition diplomats, and officials in territories controlled by the group.

It reveals huge differences in treatment of civilians, local fighters and foreigners who have joined ISIS's jihad.

A Syrian woman from occupied Mosul told of how civilian avoid the public hospitals, despite ISIS boasting of having a British-style National Health Service, funded by the state.

ISIS salary: A local fighter who has pledged allegiance to the group makes £133-£200 a month, while a foreign fighter gets £400, and can earn 'up to £666 a month, including special allowances'

As for those who have joined ISIS's jihad in Syria and Iraq, the monthly salary of local fighter who has pledged allegiance to the group is £133-£200 ($200-$300).

Meanwhile, the average salary of a foreign fighter who have travelled to join ISIS's muhajireen is reportedly nearly double that: £400($600), but they can earn 'up to £666($1,000) a month, including special allowances'.

ISIS officials have also been imposing strict taxes on its citizens, including flat-rate taxes on electricity, 'hygiene services' and use of the telephone network, as well as customs on imported and exported goods.

The militant terrorist organisation also makes money from selling oil, electricity and gas from dams and fields in Syria controlled by the militants.

Earlier this year, a report by Syrian activist group Raqqa is Being Slaughtered Silently claimed a majority of this revenue came from secret deals with the Assad regime.

And even though ISIS applies strict sharia law, they appear to have no qualms about growing and selling illegal drugs to others, with cannabis being grown on the outskirts of Raqqa to be sold on to Turkey.

Another interesting source of income is Internet cafes, which RBSS calls 'one of the most profitable trades for ISIS'.

The number of internet cafes in Raqqa had increased from 20 to 500 since ISIS came to power, fuelled by the large number of foreign fighters joining the group.