A trove of documents, seized by the American Special Operations Forces during a raid on Islamic State (Isis) leader Abu Sayyaf in Syria in May, has once again established that the Turkish army shares close links with the Isis, say reports.

The Delta Forces -- a part of the American Special Operations Forces or US Special Ops -- carried out a surprise raid in Syria in May, which resulted in the death of Isis chief finance officer Abu Sayyaf.

From Sayyaf's hideout, the US Special Ops found several documents and memory sticks, which revealed that Turkish officials were directly dealing with the Isis, according to The Guardian.

Sayyaf, who played a crucial role in running the oil and gas operations for the radical group, was apparently coordinating with Nato member Turkey for selling crude oil in the black market. The Isis makes around $10 million every month by selling oil in black. 

Also: Turkish President Erdogan's Daughter Heads 'Covert' Medical Facility for Treating Injured Isis Fighters

The Guardian report, citing US officials, stated that the documents and flash drives seized during the raid revealed links "so clear" and "undeniable" between Turkey and the Isis "that they could end up having profound policy implications for the relationship between us and Ankara."

Since 2014, Kurdish groups have repeatedly accused Turkey of having assisted the Isis. A October 2014 report citing Jordanian and Egyptian intelligence officials stated that Turkey not only provided Isis fighters with satellite data, but also trained several fighters.

The Business Insider similarly noted that Turkey cut off all ties with Syrian President Bashar al Assad and officially closed its borders, but not before opening up its southern frontier as a transit point for the Isis.

The open border helped the Isis to smuggle in arms, ammunition and even fighters, and in return, exported cheap oil to Turkey.