ISTANBUL — In the middle of an angry exchange with Russia, Turkey’s President Recep Tayyip Erdogan is accusing a Christian businessman in Syria with Russian nationality of brokering deals between the Damascus government and the jihadists of the so-called Islamic State.

Erdogan said this week that George Haswani was the “biggest trader in this.” The Turkish president asserted Haswani had dual Syrian and Russian citizenship and was buying oil from ISIS and selling it to the government of Syrian President Bashar al-Assad as well as to international traders. Erdogan said Turkey had evidence to back up the claim of Haswani’s dealing between the government and ISIS, which officially are sworn enemies.

He was reacting to charges by Russian President Vladmimir Putin that and other Russian officials that Erdogan and his family were personally involved in the illegal export of ISIS oil to Turkey. The row is part of the Turkish-Russian confrontation that started when Turkey shot down a Russian fighter plane on the Syrian border on Nov. 24.

Both sides may have a point. It is no secret that ISIS has been shipping oil out of Syria into Turkey, and it is not secret, either, that it has been selling oil to the Assad regime. But the accusations against Haswani and his kaleidoscopic connections are particularly revealing.

Haswani is a Christian from Yabroud, a city of 25,000 around 50 miles north of Damascus and close to the border with Lebanon. Turkish media are reporting that Haswani was trained in Russia and later set up a company called HESCO as an engineering and construction venture in Syria.

Erdogan is not alone in accusing Haswani. Last week, the U.S. Treasury Department put him on a blacklist, calling him “a middleman for oil purchases by the Syrian regime from ISIL” (the U.S. government’s preferred acronym for ISIS).

Haswani’s company was operating “energy production facilities in Syria, reportedly in areas controlled by ISIL,” the Treasury added. Back in March the European Union slapped sanctions on Haswani, who, Brussels says, also uses the names Al Hasawani and Heswani. The EU said he was trusted by Assad as a middleman and a link between the Syrian government and ISIS.

“The claim about Haswani paints a picture of cynical dealings between two monsters, with a Christian middleman handling their mutually beneficial business,” the Lebanon-based news website NOW reported last year. “It may well turn out that it was a Christian crony of the regime who facilitated its dealings with [ISIS] and directly contributed to the growth of the group.”

When the EU put up sanctions against Haswani and HESCO, it said he provided support for, and benefitted from, the Assad regime by acting as a middleman in deals for the purchase of oil from the Islamic State. Haswani and 12 other individuals and entities had their assets in Europe frozen and were placed under a travel ban covering all 28 EU countries.

At the time, Haswani rejected the accusations, telling Reuters that the EU did not have evidence for its claims and that Brussels should look at middlemen who were bringing ISIS oil to Turkey instead. But EU leaders said the evidence against Haswani was overwhelming.

Putting Haswani on the sanctions list was “yet another indication that Assad’s ‘war’ on ISIL is a sham and that he supports them financially,” said British Foreign Minister Philip Hammond.

The Turkish website Radikal and other news sources say that HESCO has close business links with Russian companies and the Russian military. Haswani reportedly married a Russian woman and there were rumors that he had been recruited by the Russian intelligence service. He later divorced his Russian wife and married a Syrian woman with family ties to Assad.

HESCO’s Moscow office is allegedly run by Haswani’s son–in-law, Joseph Arbash, a former member of the Syrian Communist Party. The firm has business interests in Sudan and Algeria and is a major Syrian company, according to the EU. Two years ago, the company built a gas processing plant in Syria and is reportedly a subcontractor of the Russian company Stroytransgaz.

The gas facility at Tabqa in central Syria was taken over by ISIS last year and is now run jointly by ISIS and the Syrian government, according to an EU diplomat.

News reports say Haswani was involved in negotiations leading to the release of a group of 13 Greek-Orthodox nuns in Syria in 2014. The nuns were held hostage by the Nusra Front, the al Qaeda branch in Syria, and were exchanged for a group of women and children, including relatives of Syrian opposition figures held by the government. At one point, the hostage-takers and the nuns reportedly moved into Haswani’s house in Yabroud.

Of course, Haswani and his relatives are hardly alone in all this, and the stench from these oily deals with terrorists who enslave, rape, behead, and crucify their victims can be scented in Moscow as more revelations come out.

The pointed accusations by Turkish President Erdogan had many in Russia wondering what sort of game the Kremlin really is playing in Syria, where it claims to be fighting ISIS, but spends most of its resources defending Assad.

On Friday morning, independent broadcasters Radio Echo of Moscow and Rain TV discussed “oil friendship” between the Kremlin, Assad, and ISIS that showed Putin’s war on terror in Syria in a completely new light.

Rain TV commentator Stanislav Belkovsky said he was not surprised by Erdogan's claims. “I have suspected that Putin did not bomb ISIS troops much because they helped Assad to get rid of his opponents, and now we see that Damascus and Moscow have business interests.”

While Haswani may be serving as the middleman, the alleged moneyman is Kirsan Illyumzhinov, a former president of Kalmykia, a largely Buddhist Russian republic that borders the Caspian Sea, and also the longtime president of the World Chess Federation.

Illyumzhinov was famous for his Middle Eastern connections and shadowy businesses. The U.S. Treasury reports he has particularly close links to Mudalal Khuri, who “represents [Assad] regime business and financial interests in Russia.”

In 2011, Illyumzhinov took The Daily Beast’s Anna Nemtsova on his private plane to show his home in Kalmykia. While still sober in the first half of the day, Illyumzhinov spoke about his friendship with Middle Eastern dictators. He liked them all, he said, they were all his friends.

Speaking of himself in the third person, Illyumzhinov showed Nemtsova around his “chess palace” in the little republic’s capital of Elista. As he pointed out photographs of “Mr. Illyumzhinov next to the greatest people,” he paid special attention to a portrait of his own smiling face next to Syrian leader Bashar al-Assad.

That year, 2011, Illyumzhinov toured 10 Asian and Middle Eastern countries. “I like them all the same way: Saddam Hussein, Muammar Gaddafi, the Dalai Lama, or Vladimir Putin—everybody who supports the idea of chess playing is my friend,” he told Nemtsova, apparently unconcerned that the first two were deposed.

On the same trip, Illyumzhinov told Nemtsova about his trip with aliens by UFO. “Those who have had similar experiences to mine are afraid of publicly admitting the truth, as people might think they are schizophrenic,” he told her.

By the time he and Nemtsova landed back in Moscow, Ilyumzhinov was deadly drunk. His oligarch friends worried about him as he wandered away from the plane along the highway repeating: “It is time to buy meat, I am going to buy meat.”

In 2012, Illyumzhinov met with Assad for three hours and later admitted he was willing to help the Syrian leader.Neither Illyumzhinov nor his Russian Financial Alliance Bank answered phone calls on Friday morning.

— Anna Nemtsova contributed reporting from Moscow.