Russian President Vladimir Putin on Monday accused Ankara of shooting down a Russian warplane last week to protect supplies of oil from the Islamic State group to Turkey.
"We have every reason to think that the decision to shoot down our plane was dictated by the desire to protect the oil supply lines to Turkish territory, right to the ports where it is loaded onto tankers," Putin said during a news conference on the fringes of UN climate talks near Paris.
"We have received additional information which unfortunately confirms that this oil, produced in areas controlled by the Islamic State and other terrorist organizations, is transported on an industrial scale to Turkey."
After the Su-24 bomber was downed on the Syrian border last week, Putin accused the Turks of being "accomplices of terrorists" and said oil from ISIS territory was being exported through Turkey.
Revenues from selling oil constitute one of the main sources of income for ISIS, which controls large areas of Syria and Iraq.
Just four days ago, Turkish President Reccep Tayyip Erdogan denied Russian accusations that Ankara was collaborating with ISIS, saying his country's fight against the jihadists was undisputed.
"Our country's stance against Daesh has been clear since the very beginning," Erdogan said in a speech to local officials at his presidential palace in Ankara. "There is no question mark here. Nobody has the right to dispute our country's fight against Daesh or to incriminate us."
But speculation over possible Turkish involvement with ISIS has loomed large since a senior Western official leaked details of a US special forces operation in May to kill Islamic State (ISIS) leader Abu Sayyaf, wherein he said evidence was found that Turkey was one of the main buyers of ISIS's oil.
AFP contributed to this report.