The Islamic State militant group brings in up to $50 million each month in oil sales, allowing the militants to prolong their control over large swathes of Iraq and Syria, the Associated Press reported Friday. Iraqi and U.S. officials, some of whom spoke to the AP on condition of anonymity, said regional actors were intentionally or unintentionally aiding the group, also known as ISIS, even as a U.S.-led coalition sought to cut off ISIS's oil revenue.

Oil sales account for the militant group’s largest source of continued funding and was thought to have allowed the group to rebuild infrastructure, pay fighters in exchange for loyalty and continue funding their military endeavors. 

ISIS sells crude to smugglers at prices that vary between $10 and $35 per barrel, Iraqi intelligence told the Associated Press. The international price of oil is about $50 per barrel. The smugglers then sell the oil to middlemen in Turkey, or, according to one Iraqi official, to individuals in Kurdish areas of Iraq, as well. A Kurdish politician denied the claim. ISIS was believed to extract about 30,000 barrels a day in Syria, and about 10,000 to 20,000 barrels per day in Iraq.

A U.S.-led airstrike campaign has sought to target ISIS’s ability to generate money in recent months, as the group has continued to make gains in Syria and Iraq. The coalition carried out a large-scale attack Thursday on Syria’s Omar oil field. The refinery was believed to generate between $1.7 and $5.1 for the group.

Turkey’s prime minister’s office issued a statement to the Associated Press saying that it had taken measures to strengthen border security and has prevented oil smuggling across its border. As of the end of September, its security forces had stopped 3,319 cases of smuggling from Syria, the statement said.

The group also imposes a range of taxes in areas under its control that were believed to bring in considerable profit. In addition, they have made money through looting as they have expanded through Iraqi and Syrian cities. ISIS rules more than about a third of Iraq and Syria. The group has imposed a strict penal code in areas under its control, as it threatens to expand further throughout the region.