A war of words has broken out between Russia and Turkey over accusations that Turkey is buying oil smuggled by Islamic State (IS).

Key points: US Treasury official says Islamic State brings in $500 million per year in oil revenue

US Treasury official says Islamic State brings in $500 million per year in oil revenue Security analyst says there is huge black market for IS oil in the West

Security analyst says there is huge black market for IS oil in the West Former Iraqi MP backs up claims by Turkish opposition MP that IS smuggled $800 million worth of oil into Turkey from Syria and Iraq

Russian president Vladimir Putin has accused Turkey of shooting down a Russian warplane last week out of a desire to protect these oil supply lines.

"At the moment we have received additional information confirming that oil from the deposits controlled by Islamic State militants enters Turkish territory on an industrial scale," he said on the sidelines of the climate change summit in Paris.

"We have every reason to believe that the decision to down our plane was guided by a desire to ensure security of this oil's delivery routes to ports where they are shipped in tankers."

Turkish president Tayyip Erdogan has attacked the claims as slander, and said if there was proof that Turkey was co-operating with IS he would resign.

"We are not that dishonest as to buy oil from terrorists. If it is proven that we have, in fact, done so, I will leave office," he was quoted by Russia's TASS news agency as saying.

"If there is any evidence, let them present it, we'll consider [it]."

So is there evidence Turkey is buying oil smuggled by IS?

It is no secret that IS rakes in a fortune from oil fields it now controls in Syria and Iraq. A US treasury official recently told a security conference in Aspen that IS brought in $500 million a year in oil revenues.

"Earlier this year [IS] made about $40 million in one month off of the sale of oil. So if you want to extrapolate that out, you can get to about $500 million in the course of a year, and this is all internally generated," David Glaser, assistant secretary of the US treasury for terrorist financing, said.

Security analysts say Western black market exists for IS oil

In September 2015, IS seized the last major oilfield under Syrian government control in a desert area north-west of the ancient city of Palymra and a region that holds Syria's main natural gas fields and multi-million dollar extraction facilities.

Since taking control of key areas of Syria and Iraq last year, IS has developed an illicit trade in oil sales.

Many security analysts believe there is a huge black market for IS oil in the West.

"Islamic State's barbaric conquests are financed by collaborators among the business and financial circles of the West," wrote Dr Norman Bailey, of the Centre for National Security Studies and Geostrategy at Haifa University in Israel.

"Buyers are readily found in the West and tanker trucks are permitted to reach ports where oil tankers are waiting to load it.

"All this requires ready, willing and able collaborators among the business and financial circles of the West as well as surrounding countries."

Dr Bailey was reluctant to name the ports or countries involved.

"This is not the place to name names, but the buyers and the facilitators and the financiers are not that difficult to identify, so that if the opponents of Islamic State were really serious about doing something about it, along with the occasional bombing raid, the IS could be starved of funds," he wrote.

Turkish opposition politician claimed IS smuggled $800m of oil

In June 2014, a member of Turkey's parliamentary opposition, Ali Edibogluan, claimed that IS had smuggled $800 million worth of oil into Turkey from Syria and Iraq, according to the Al Monitor website.

He cited oil fields at Rumaila in northern Syria and others near Mosul in Iraq, saying that IS had laid pipes allowing it to "transfer the oil to Turkey and parlay it into cash".

"Turkey's cooperation with thousands of men of such a mentality is extremely dangerous," he said, according to the Al Monitor report.

"Similar pipes exist also at [the Turkish border regions of] Kilis, Urfa and Gaziantep ... they take the oil from the refineries at zero cost.

"Using primitive means, they refine the oil in areas close to the Turkish border and then sell it via Turkey."

Now, a former Iraqi member of parliament has backed up those claims.

"In the last eight months [IS] has managed to sell what is $800 million worth of oil in the black market of Turkey. This is Iraqi oil and Syrian oil, and these are carried by trucks from Iraq, from Syria, through the borders to Turkey and sold ... [at] less than 50 per cent of the international oil price," Mowaffak al Rubaie said in an interview with the Russian channel RT.

"It has always been sold in the region of $21-22 for the barrel.

"Now this either gets consumed inside, the crude is refined on Turkish territory by the Turkish refineries, and sold in the Turkish market. Or it goes to Jihan and then in the pipelines from Jihan to the Mediterranean and sold to the international market.

"Money and dollars generated by selling Iraqi and Syrian oil on the Turkish black market is like the oxygen supply to [IS] and its operation.

"Once you cut the oxygen then [IS] will suffocate."

'No shadow of a doubt' Turkey knew about smuggling

The former Iraqi MP said there was "no shadow of a doubt" that the Turkish government knew about the smuggling operations.

"The merchants, the businessmen [are buying oil] in the black market in Turkey under the noses, under the auspices if you like, of the Turkish intelligence agency and the Turkish security apparatus," he told RT.

Mr Rubaie also accused Turkey of providing medical treatment to IS terrorists in Turkish hospitals along the border with Syria and even in "Istanbul itself".

"There are security officers who are sympathising with [IS] in Turkey," he said.

"They are allowing them to go from Istanbul to the borders and infiltrate ... Syria and Iraq.

"There is no terrorist organisation which can stand alone, without a neighbouring country helping it — in this case Turkey."

The Iraqi politician urged Turkey to come clean and join the international efforts to destroy IS.

Turkish president Tayyiip Erdogan (L) has labelled Russian president Vladimir Putin's accusations as "slander". ( Reuters: Ivan Sekretarev, file photo )

The Russian news outlet said Mr Putin himself noted that it was "hard to believe, but is theoretically possible" that the Turkish leadership knew nothing about oil flowing into Turkey illegally.

"Vehicles, carrying oil, lined up in a chain going beyond the horizon," Mr Putin reportedly said as he compared the views seen by Russian pilots and drones to a "living oil pipe" stretching from IS and rebel-controlled areas in Syria into Turkey.

"Day and night they are going to Turkey. Trucks always go there loaded, and back from there — empty."

The RT website has aerial photographs that appear to show rows of trucks near the Turkish-Syrian border.

Turkey's president Mr Erdogan has called claims that Turkey buys oil from IS "slander".

"I will say something very strong here. If such a thing is proven, the nobility of our nation would require that I would not stay in office," he said.

"The countries from which Turkey buys oil are well known."

Russia has imposed a series of economic and trade sanctions against Turkey in retaliation to the decision to shoot down the Russian war plane.