In an oil field in northeastern Syria, a line of trucks lines up daily to load crude sold cheaply by Islamic State militants who have hijacked parts of the country's energy industry in their bid to build a caliphate.

Sales at Shadada field, described by an oil trader, are just one example of how the group, which has seized land in war-torn Syria and neighbouring Iraq, is creating its own economy through a series of pragmatic trades.

It is cutting deals with local traders and buyers, even businessmen who support Syrian President Bashar Assad, and some of its oil has made its way back to government buyers through a series of middlemen.