Now the violently anti-Western jihadist group, which has been steadily extending its control in the region, is selling the crude oil to local entrepreneurs, who use home-made refineries to produce low-grade petrol and other fuels for Syrians facing acute shortages.

The ability of Jabhat al-Nusra to profit from the oil locally, despite international sanctions which have hindered its sale abroad, will be particularly worrying to the European Union, which has voted to ease the embargo but at the same time wants to marginalise the extremist group within the opposition.

In the battle for the future of the rebel cause, the oil-fields may begin to play an increasingly strategic role. All are in the three provinces closest to Iraq – Hasakeh, Deir al-Zour, and Raqqa, while the Iraqi border regions are the homeland of the Islamic State of Iraq, as al-Qaeda’s branch in the country calls itself.