Russia's defence ministry on Saturday accused the United States of turning a blind eye to the trafficking of oil into Turkey from Syrian areas under Islamic State control, after Washington called the amounts involved insignificant.
"When US officials say they don't see how the terrorists' oil is smuggled to Turkey... it smells badly of a desire to cover up these acts," the ministry said on its Facebook page.
"The declarations of the Pentagon and the State Department seem like a theatre of the absurd," the statement added, suggesting that Washington "watch the videos taken by its (own) drones which have recently been three times as numerous over the Turkey-Syria border and above the oil zones".
US special envoy and coordinator for international energy affairs, Amos Hochstein, on Friday said the amount of oil smuggled into Turkey from areas of Syria controlled by the Islamic State group is "of no significance from a volume perspective -- both volume of oil and volume of revenue".
His comments came after Moscow accused Ankara of profiting from the trade.
Russia and Turkey have in recent days traded allegations that they are involved in the illegal trade, further ratcheting up tensions after Turkish jets downed a Russian bomber on the Syrian border.
The State Department has dismissed Moscow's charge against its NATO ally, which directly implicated President Recep Tayyip Erdogan and his family in the trade, insisting there is no evidence to support it.
"I don't believe that there is significant smuggling, between ISIS-controlled areas and Turkey of oil in any significance in volume," Hochstein said Friday, using an alternative name for IS.
Instead, US officials told reporters, the oil pumped in eastern Syria is refined in ad hoc desert pits equipped with crude stills and sold on the war zone black-market within Syria and neighbouring Iraq.
Allied officials estimated the ISIS group's income from oil at $1.0 million to $1.5 million per day, but hope that renewed US, British and French air strikes have cut that.