"It is not possible to starve an organised terrorist group because it is number one a state, so it levies taxation inside the territory that is controlled by Islamic State. But also the activity, for example the smuggling of oil, is taking place in an area which is a war zone. So of course people will buy that oil because they don't have any alternative. So even if we wanted to stop that kind of trade, it will be very, very difficult to do that."