Russia's defence ministry said on Wednesday it had proof that Turkish President Tayyip Erdogan and his family were benefiting from the illegal smuggling of oil from ISIS-held territory in Syria and Iraq.
Moscow and Ankara have been locked in a war of words since last week when a Turkish air force jet shot down a Russian warplane near the Syrian-Russian border, the most serious incident between Russia and a NATO state in half a century.
In a briefing in Moscow, defence ministry officials displayed satellite images which they said showed columns of tanker trucks loading with oil at installations controlled by ISIS in Syria and Iraq, and then crossing the border into neighbouring Turkey.
The officials did not specify what direct evidence they had of the involvement of Erdogan and his family, an allegation that the Turkish president has vehemently denied.
"Turkey is the main consumer of the oil stolen from its rightful owners, Syria and Iraq. According to information we've received, the senior political leadership of the country - President Erdogan and his family - are involved in this criminal business," said Deputy Defence Minister Anatoly Antonov.
"Maybe I'm being too blunt, but one can only entrust control over this thieving business to one's closest associates."
"In the West, no one has asked questions about the fact that the Turkish president's son heads one of the biggest energy companies, or that his son has been appointed energy minister. What a marvellous family business!"
"The cynicism of the Turkish leadership knows no limits. Look what they're doing. They went into someone else's country, they are robbing it without compunction," Antonov said.
The ministry also alleged that the same criminal networks which were smuggling oil from ISIS-held areas into Turkey were also supplying weapons, equipment and training to the militant group.
Officials said that the Russian air force's bombing campaign had made a significant dent in ISIS ability to produce, refine and sell oil.
Erdogan last week denied that Turkey procures oil from anything but legitimate sources. He has said Ankara is taking active steps to prevent smuggling of fuel, and he challenged anyone who accused his government of collaborating with ISIS to prove their allegations.