The jihadist Islamic State is selling oil and liquid gas products extracted from fields under its control in Syria to Iraqi businessmen across the border, a monitoring group said Monday.

IS 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 Syrian Observatory for Human Rights.

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

The Observatory said the IS 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 its civil war.

The revolt, demanding President Bashar al-Assad's ouster, morphed into a war after the regime unleashed a brutal crackdown on dissent.

Months into the conflict, jihadists started pouring into Syria. They have been accused of committing some of the war's worst atrocities.