Relations between Russian President Vladimir Putin and his Turkish counterpart hit a new low Monday after Putin accused Turkey of shooting down one of its warplanes last week in order to protect the transit of oil sold by Islamic State through Turkey.

On Monday, Putin said Russia had proof that the SU-24 warplane downed last week by the Turkish military on the border with Syria, was done to aid the transit of that oil.

“We have just received additional information proving that unfortunately, large volumes of oil, industrial volumes coming from oil fields controlled by ISIS and other terrorist organizations, enter Turkey’s territory,” he said, according to a transcript of his speech on the Kremlin’s website.

“And we have every reason to believe that the decision to shoot down our plane was dictated by the desire to ensure the security of these oil supply routes to Turkey – to the ports where it is shipped out in tankers – while protecting the Turkmen is merely a pretext.”

Speaking in Paris on Monday, President Recep Tayyip Erdogan said he would resign if there is proof of Turkey’s cooperation with Islamic State, the terror group which operates in swathes of Syria and Iraq.

“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. If there is any evidence, let them present it, we’ll consider [it],” he said, as quoted by the Russian TASS news agency.

Read the article “Putin Accuses Turkey of Aiding ISIS Oil Trade, Erdogan Denies Charge” on cnbc.com.