Russian President Vladimir Putin, during the climate talks in Paris, said that Moscow has evidence showing that Turkey shot down the Russian Su-24 fighter jet last week to protect illicit oil shipments from the Islamic State (ISIS/ISIL).

“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,” Putin said on Monday, according to Russian news outlet Sputnik News.

The Russian president, discussing the matter with world leaders during the climate talks, said that most members of the international community agree there was no need to attack the aircraft because it was not a threat to Turkey.

He referred to the incident as a “huge mistake” on Turkey’s part.

“We have heard from the Turkish side that this decision was not made by the president, it was made by other people. For us, it does not matter much, the important thing is that two of our servicemen died as a result of this criminal move,” Putin told reporters.

Turkish President Recep Tayyip Erdogan has denied Putin’s accusations that Turkey is purchasing oil from ISIS, calling the Russian leader’s comments “slander.”

“We buy from Russia, Iran, Azerbaijan, Qatar, and Nigeria. We won’t accept such slanders,” Erdogan told reporters in Paris.

If the allegations by Russia are proven to have merit, the Turkish leader said he would resign from office.

“The accusation that Turkey allegedly buys crude oil from the Islamic State is unacceptable, and to say it is amoral. You can’t just say things, you need to present evidence,” Erdogan said. “If documents exist — let’s see them. If this fact is proven, I will not stay in my position.”

The conflict in Syria is expected to be the main topic of bilateral discussions during the climate summit, said Putin, adding the he hopes Russia’s counterterrorism cooperation with other countries, particularly France, will “go further.”

“He said that despite the downing of the Su-24 by Ankara, Moscow will continue to strive for a broad, international coalition to combat the terrorist group, adding that he regrets the deterioration of relations between Russia and Turkey,” reports Sputnik.

“I think that this is regrettable for all of us,” the Russian president said. “And for me personally it’s a real pity, because I myself did much to build up relations with Turkey over the course of a long period of time.”