The US-led coalition against Islamic State has targeted 12 oil refineries in Syria during a third night of air strikes against themilitants.
The raids - carried out by US, Saudi and UAE aircraft - killed 14 IS fighters and five civilians in eastern Syria, activists said.
The US military said the refineries generated as much as $2 million (£1.2m) per day in revenue for the militants.
US President Barack Obama has vowed to dismantle the IS "network of death".
IS has seized large areas of Syria and Iraq in recent months and controls several oilfields. Sales of smuggled crude oil have helped finance its offensive in both countries.
The US has launched nearly 200 air strikes against the militants in Iraq since August and expanded the operation against IS to Syria on Monday.
'Successful strikes' The US military said the latest strikes, using fighter jets and drones, hit "small-scale" refineries that were producing "between 300-500 barrels of refined petroleum per day".
"We are still assessing the outcome of the attack on the refineries, but have initial indications that the strikes were successful," the US Central Command said in a statement.
Pentagon spokesman Rear Adm John Kirby said the purpose of the strikes was "not necessarily to kill militants" but to destroy the facilities, which were funding IS through the black market.
The UK-based Syrian Observatory for Human Rights, an activist group that monitors the Syrian conflict, said the strikes killed 14 IS fighters in Deir al-Zour and five civilians in Hassakeh.
Kurdish forces in northern Syria say they have pushed back an advance by IS fighters near the Kurdish town of Kobane, close to the border with Turkey.
Local media said clashes in the area were continuing on Thursday.