Who is buying oil from Islamic State?

Russia and Turkey's war of words is not calming. The initial disagreement was over whether the Russian plane shot down on November 24 was in Turkish airspace.

Now Russia's President Putin says that Turkey is involved in Islamic State's illegal oil trade.

Turkey denies it trades with Islamic State militants. Turkey's President Erdogan has challenged Mr Putin to prove his allegations.

The BBC's Tom Heyden looks into Islamic States oil reserves and who buys from them.