New York: ISIS made a whopping USD 2.4 billion in 2015 despite losing territory by simply adjusting its business from oil to taxes and is still the richest terror group on the planet, according to a report.

The new report, by the Center for the Analysis of Terrorism on the finances of the Islamic State, said that despite the constant airstrikes on its oil infrastructure, ISIS still has an over USD 2 billion empire and it is making up lost revenue by squeezing about 8 million people under its control through raising of taxes.

The authors of the report, terrorism experts Jean-Charles Brisard and Damien Martinez, conclude that "ISIS's military defeat is not imminent...As things stand, ISIS economic collapse remains some way off in the mid-term."

The report says ISIS made USD 2.4 billion in 2015. That is a USD 500 million drop from the center's revenue estimate the previous year, but ISIS remains the richest terrorist organisation on the planet.

The Islamic State's extortion of the people living inside its territory in Iraq and Syria has skyrocketed from USD 360 million in 2014 to USD 800 million in 2015, according to researchers.

The theoretical value of assets under ISIS control (oil reserves, gas reserves, minerals, cash assets) was estimated at USD 2,260 billion by the end of 2015, up by 11 per cent compared to late 2014, the report released yesterday said.

"It's really an adaptive organisation. What strikes me is the fact that they're clearly behaving as managers, not simple looters. They really have budget requirements, and they're compensating," Brisard told CNN Money.

The report is a sobering take on what has been an image of ISIS as a terrorist organisation in disarray. In recent months, ISIS has cut fighter salaries in half and a nonstop bombing campaign has cut its oil production.

It has also lost 40 per cent of its territory, according to the US military, but ISIS still has about 8 million people under its control.

Brisard and Martinez were quoted as saying that ISIS has simply adjusted its business from oil to taxes. As oil fell from 38 per cent to 25 per cent of its revenue stream last year, ISIS cranked up its extortion racket.

In 2015, taxes went from supplying 12 per cent of yearly ISIS revenue to 33 per cent.

Those taxes include a 10 per cent income tax, up to 15 per cent business tax, road tolls, 5 per cent fees for bank cash withdrawals and up to 35 per cent taxes on pharmaceutical drugs.

There are also fees for leaving the territory, even temporarily. And there's a special tax on non Muslims called jizyah.

However, the US Treasury Department said the coalition's effort to disrupt the Islamic State's economy is working.

"We are seeing progress... Since late-2015, ISIL's production of oil has declined by about 30 per cent. Their ability to generate revenue has been reduced by at least that much," the Treasury was quoted as saying.