The Syrian regime of Bashar al-Assad has negotiated multimillion-dollar oil deals with the Islamic State militant group (ISIS), new documents retrieved from a U.S. raid against the radical Islamists’ oil chief have revealed.

A U.S. special forces raid on ISIS’s oil minister—known by his nom de guerre Abu Sayyaf—in May 2015 saw the extraction of thousands of documents implicating his oil operation with that of the Assad regime, with revenue from the sale of oil to Damascus helping the group to reach a peak of $40 million a month in oil revenue, according to documents seen by The Wall Street Journal.

Sayyaf led the group’s oil ministry, known as the Diwan of Natural Resources, and used deals with the Syrian regime to boost the division’s income, contributing 72 percent of $289.5 million the group earned in natural-resource revenues in the six months preceding February 2015.

A document identified as Memo No. 156, dated February 11, 2015, from the trove of documents extracted from Sayyaf’s Deir Ezzor hideout shows that the Tunisian national requested assistance from an unknown party on how to build investment links with businessmen allied to the Assad regime.