Russian officials have reportedly accused the president of Turkey and his family of a direct tie to the Islamic State, claiming Tayyip Erdogan and his family are linked to the group’s oil smuggling.

Multiple media outlets reported the accusation Wednesday, saying Erdogan denied the notion.

According to Russian television network RT, the country’s defense ministry briefed the media on its investigation into the Islamic State’s oil activities and funding. The country’s deputy defense minister Anatoly Antonov laid out some of the evidence, claiming it directly implicates Turkey.

RT reported the evidence included clips of Russian airstrikes on ISIS oil interests, like refineries and pumping stations. The ministry reportedly claimed those strikes, and the bombing of more than 1,000 oil trucks, have cut the Islamic State’s daily income from $3 million to $1.5 million, but a significant amount of ISIS’ stolen oil was still entering Turkey via three smuggling routes.

“Turkey is the main destination for the oil stolen from its legitimate owners, which are Syria and Iraq,” Antonov said at the media briefing, according to a NBC News report. “Turkey resells this oil. The appalling part about it is that the country’s top political leadership is involved in the illegal business — President Erdogan and his family.”

But the BBC reported that Russia did not provide any concrete proof of Erdogan’s involvement at the briefing. Russia said it was only showing a portion of the evidence Wednesday, and RT reported that the ministry would be providing additional material on its website at a later date.

Erdogan reportedly dismissed the claims as slanderous.

“No one has the right to slander Turkey, especially the slander of Turkey buying ISIS oil,” CNN quoted Erdogan as saying during a speech at Qatar University. “Turkey has not lost its moral values to buy oil from a terror organization.”

Tensions between the two countries have been high since a Turkish plane shot down a Russian plane near the Syrian border last Tuesday. The two countries have disputed the circumstances of the incident, with Turkey saying the Russian plane ignored repeated warnings and crossed into its airspace and Russia claiming no warnings were given and Turkish airspace was not violated.

Earlier this week, CNN reported Russian President Vladimir Putin accused Turkey of shooting down the plane to keep its ISIS oil activity hidden. Erdogan vowed to resign if that was proven.

Russia has imposed some economic and travel sanctions on Turkey for its role in the incident.

President Barack Obama has asked the two nations to reduce tensions.

The United States has also asked Turkey to seal off its border with Syria to prevent Islamic State insurgents from shuffling oil and manpower, and U.S. Secretary of State John Kerry told CNN on Wednesday that Turkey plans to shut down the portion of its border that’s still not secure.