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", Reuters reported.

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, Reuters notes.

Iraqi officials say that in recent weeks the group has transported oil from Qayara to be processed by mobile refineries in Syria into low quality gasoil and gasoline, then brought back for sale in Mosul, a city of 2 million people.

Larger shipments of crude, some of them from Najma, are also sold via smugglers to Turkish traders at vastly discounted prices of around $25 per barrel, they said.

"We have confirmed reports showing that the Islamic State is shipping crude from Najma oilfield in Mosul into Syria to smuggle it to one of Syria's neighbors," said Husham al-Brefkani, head of Mosul provincial council's energy committee.

"The Islamic State is making multi-million dollar profits from this illegal trade."

Najma and Qayara had been operated by Angola's state-owned firm Sonangol, but it pulled out last year declaring force majeure amid rising development costs and security concerns over Sunni militants in the area, even before last month's assault.

Qayara, which has estimated reserves of 800 million barrels, had been producing 7,000 barrels per day of heavy crude before the Islamic State took over the field and a nearby 16,000 bpd refinery. Qayara refinery and second smaller plant at Kasak, northwest of Mosul, stopped operating when staff fled.

But Qayara oilfield itself has kept pumping after the militants asked Iraqi employees to stay at their posts, promising to protect them - as they have done at most oil facilities in order to maintain production.

Iraqi government sources said it was hard to assess how much money the group makes from selling crude or the fuel refined in Syria as the number of trucks fluctuates daily. One source said that a separate - and now terminated - smuggling operation into the Kurdish enclave and into Iran generated nearly $1 million a day earlier this month.