ISIL has captured large swathes of territory in Syria and Iraq, including all of oil-rich Deir Ezzor province's oil fields in eastern Syria.

"Trucks with Iraqi number plates have in the past few days travelled to Deir Ezzor's oil fields from Iraq, to fill up and transport oil towards western Iraq," said the so-called Syrian Observatory for Human Rights on Monday.

The Britain-based monitoring group added: "These trucks belong to Iraqi businessmen who came (to Syria) to buy oil from fields under ISIL control."

Observatory director Rami Abdel Rahman said "a considerable number of trucks" had been sighted, travelling from Syria into Iraq each day.

"Each barrel of oil is sold to Iraqi businessmen for $20 to $40," Abdel Rahman told AFP. Much of Syria's border with Iraq is under control of ISIL terrorists.

The Observatory said ISIL was also selling oil to Syrians living in areas under their control for $12 to $18, "to draw the support of the local population."

Oil is sold at more than $100 per barrel on global markets.

Syria's official oil production has dropped by 96 percent since the March 2011 outbreak of the war.

Months into the conflict, foreign-backed terrorists started pouring into Syria. They committed some of the war's worst atrocities.

BA/BA