The US-led coalition has bombed oil refineries in Syria, as part of a series of airstrikes targeting militant group, the Islamic State.

In targeting oil facilities, the US wants to hit the militants where it really hurts – right in the wallet.

Oil is crucial for the Islamic State. Firstly, because it's refined and used as fuel, either for themselves or the civilians that live in their self-declared caliphate.

But primarily, oil's a source of revenue. The group sells the refined crude on the black market in Syria, or through middlemen who then smuggle it over the border into Turkey. In the past it has even sold oil to Bashar al-Assad's regime in Damascus, the guys they are meant to be fighting against.

It's hard to know how much cash this generates for the militants, but recent estimates range from a million dollars per day, to two million dollars a day.

That kind of money means Islamic State can sustain itself economically - it doesn't need the backing of wealthy foreigners, and the demands that often accompany donations.

Stopping the oil flow means stopping the cash flow, which would leave the Islamic State without a key source of revenue.