ISIS (Islamic State) employed more than 2,000 workers at its peak as it looked to make millions of dollars from oil in Syria and Iraq.
According to analysis by the Iraq Oil Report, an intensifying number of US air strikes has helped to shut down almost all of the terrorist groups operations in the country.
The strikes have also hindered its oil business in Syria.
Previous estimates have shown ISIS has been able to generate as much as $40million per month from the sale of crude oil.
It is believed around 1,600 of those working for IS had come from other countries.
US State Department officials said redundancies within the global oil and gas industry had helped recruitment efforts.
Amos Hochstein, the State Department’s Special Envoy for International Energy Affairs, told the Iraq Oil Report that results from Operation Tidal Wave II had already been seen.
Operation Tidal Wave II began in October in a bid to target the group’s oil sector.
Earlier this year Energy Voice spoke to an international terrorism expert who said oil and gas engineers are being told to “name their price” when applying for jobs with ISIS.