In a report published on Sunday, The Guardian quoted an unnamed senior Western official as saying that evidence on direct dealings between Turkish officials and ranking ISIL members was “undeniable.”

According to the Western official, the documents on the undeclared alliance were obtained following a US raid on the compound of the key ISIL figure, Abu Sayyaf, in Syria in May, which led to his killing.

“There are hundreds of flash drives and documents that were seized there,” the official said. “They are being analyzed at the moment, but the links are already so clear that they could end up having profound policy implications for the relationship between us and Ankara.”

The ISIL operative was responsible for smuggling oil from fields in eastern Syria. The oil then found its way into the black market to become the main driver of revenues for ISIL, with Turkish buyers as its main clients.

Turkey has been facing criticism for facilitating militants’ border crossings to join ISIL in Syria, which has been grappling with foreign-backed militancy since March 2011. Ankara has also been criticized for providing assistance to Takfiri terrorists waging war in the Arab country.

The report comes against the backdrop of airstrikes conducted by the Turkish military against targets controlled by Takfiri ISIL terrorists inside Syria as well as the positions of the Kurdistan Workers’ Party (PKK) in northern Iraq.