Islamic State (Isis) is banking as much as $3m (£1.98m) a week from oil sales to smugglers and the local market as US and British air strikes attempt to destroy the infrastructure that allows Daesh (IS) to pump and refine crude in its strongholds in eastern Syria.

Tom Keatinge, director of the Centre for Financial Crime and Security Studies (CFCS) at the Royal United Services Institute (Rusi) think tank in London, said that the oil produced by IS was either being trucked across the border to Turkey, sold to the regime of Bashar al-Assad or sold to Syrians living under IS rule.

His comments come amid a war of words between Russian President Vladimir Putin and his Turkish counter-part, Reccip Tayyip Erdogan, over allegations by Russia that Turkey is turning a blind eye to oil smuggling from IS-controlled territory. Erdogan has said he would resign if there proved to be evidence of Turkish government collusion in IS smuggling, which he has angrily denied.

But whether the oil is being smuggled into Turkey or not, targeting IS infrastructure remains the only feasible way for the anti-IS coalition to cut funding for the terrorist group in Syria. IS makes most of its money either by selling oil, extorting money from its population through taxes and, on a far smaller scale, selling historical artefacts and cigarettes outside of its territory.

"The only real source of funding that the international community has any reasonable control over is the oil. So bombing the oil facilities seems like the only solution when it comes to disrupting their finances," said Keatinge, adding that he was surprised the decision to launch air strikes against IS had taken so long. "I'm not quite sure why two weeks ago the bombing of oil fields and oil infrastructure started to become popular. They've been there for the last 18 months, it is not like we have only just discovered them."