The Syrian Government cut deals with Isis (Islamic State) to help the jihadists earn more than US$40 million a month from the sale of oil, documents recovered from a US and British raid on a key commander have revealed.

Thousands of spreadsheets and accounts kept by the group's oil boss, Abu Sayyaf, retrieved in the biggest intelligence raid in US Special Forces' history last year, reveal how the two sides forged a mutually beneficial arrangement despite being at war with one another.

Fighters from Isis captured some of the state's best-producing oil fields in eastern Syria in 2013.

The Daily Telegraph reported claims two years ago that the regime had been purchasing oil from the jihadists, but the documents, seen by the Wall Street Journal, show the scale of the collusion.

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At the height of production in late 2014 to early 2015, Isis recorded US$40.7 million profit each month - most of which was made from sales to the Syrian Government, according to the US Treasury Department.

One memo, dated February 11 2015, sent from Isis' treasury to Abu Sayyaf's office, requested guidance on establishing investment relationships with businessmen linked to Bashar al-Assad's regime.

The document cited agreements allowing lorries and pipeline transit from government-controlled fields through Isis-controlled territory. Known only by his nom de guerre, Abu Sayyaf, a Tunisian who moved to Iraq after Saddam Hussein's fall, became close to a number of senior Sunni militants during the US invasion, including Abu Bakr al-Baghdadi, the founder of Isis.

After Baghdadi established his "caliphate" in June 2014, Abu Sayyaf was given the job of operating its oil business from headquarters in the al-Omar field in Deir Ezzour near the Iraqi border, previously run by the Anglo-Dutch company Royal Dutch Shell.

The documents reveal how instead of getting rid of state employees at the fields they took over, Abu Sayyaf offered them handsome salaries to stay - sometimes up to four times the national rate. According to the accounts of people who worked for him, he was a much-feared boss who would threaten his 152 employees with beheadings or exile to Iraq if they disobeyed him.

He became instrumental in helping it become the world's wealthiest terrorist group. His division reported US$40.7 million in revenue in October-November 2014, a near 60 per cent rise on the previous month.

Spreadsheets retrieved show Isis' natural resource revenues in the six months that ended in February 2015 amounted to US$289.5?million - 70 per cent of which came from Abu Sayyaf's oil fields.

The huge profits continued until Abu Sayyaf's death in a raid by US Special Forces and the SAS on his home in Deir Ezzour in 2015.