(A fighter from the Islamic State, formerly known as the Islamic State of Iraq and the Levant (ISIL), mans an anti-aircraft gun mounted on the rear of a vehicle in Mosul July 16, 2014. The Iraqi army and allied Shi'ite militia forces are trying to push back the Sunni insurgents of the al Qaeda offshoot, who swept through northern Iraq last month to within 70 km (45 miles) of Baghdad. The banner on the bridge reads: "Welcome to the State of Nineveh; There is no God but God and Mohammad is the Messenger of God". Picture taken July 16, 2014. REUTERS/Stringer )

(A fighter from the Islamic State, formerly known as the Islamic State of Iraq and the Levant (ISIL), mans an anti-aircraft gun mounted on the rear of a vehicle in Mosul July 16, 2014. REUTERS/Stringer )

Islamic State militants seized four small oilfields when they swept through north Iraq last month and are now selling crude oil and gasoline from them to finance their newly declared “caliphate”.

Near the northern city of Mosul, the Islamic State has taken over the Najma and Qayara fields, while further south near Tikrit it overran the Himreen and Ajil fields during its two-day sweep through northern Iraq in mid-June.

The oilfields in Islamic State hands are modest compared to Iraq’s giant fields near Kirkuk and Basra, which are under Kurdish and central government control. Most of the Islamic State-held oil wells – estimated by a Kurdish official to number around 80 – are sealed and not pumping.

But the monopoly over fuel in the territory it has captured gives the Islamic State leverage over other armed Sunni factions who could threaten its dominance in northern Iraq.