The so-call said militants had looteed Islamic State (IS) has made more than $500m trading oil, a US treasury official has said.

Its "primary customer" has been the government of Syria's President Bashar al-Assad, despite its ongoing battle to overthrow the regime, Adam Szubin told the BBC. IS had also looted up to $lbn from banks in territory it held, he said.

After Turkey downed a Russian fighter jet last month, Russian President Vladimir Putin said he had intelligence that large amounts of oil and petroleum products were moving across the border from Islamic State territories to Turkey. Turkey has denied the accusation furiously.