U.S.-led coalition warplanes bombed oil installations and other facilities in territory controlled by Islamic State militants in eastern Syria on Friday, taking aim for a second consecutive day at a key source of financing that has swelled the extremist group's coffers, activists said.

The strikes hit two oil areas in Deir el-Zour province a day after the United States and its Arab allies pummeled a dozen makeshift oil producing facilities in the same area near Syria's border with Iraq. The raids aim to cripple one of the militants' primary sources of cash — black market oil sales that the U.S. says earn up to $2 million a day.

The Britain-based Syrian Observatory for Human Rights said the strikes overnight and early Friday hit the Tink oil field as well as the Qouriyeh oil-producing area in Deir el-Zour. It said air raids also targeted the headquarters of the Islamic State group in the town of Mayadeen.

The Observatory said the strikes were believed to have been carried out by the coalition. Another activist collective, the Local Coordination Committees, also reported four strikes on Mayadeen that it said were conducted by the U.S. and its allies.

The Observatory, which relies on a network of activists inside Syria, said there were reports of casualties in the strikes, but did not have concrete figures.

The Observatory reported another apparent coalition air raid on Islamic State positions outside the city of Hassakeh in northeastern Syria near the Iraqi border.

The strikes there targeted another oil production area, as well as vehicles the militants had brought in from Iraq and tried to bury in the ground to protect them, according to Observatory director Rami Abdurrahman.

The U.S.-led coalition, which began its aerial campaign against Islamic State fighters in Syria early Tuesday, aims to roll back and ultimately crush the extremist group that has created a proto-state spanning the Syria-Iraq border. Along the way, the militants have massacred captured Syrian and Iraqi troops, terrorized minorities in both countries and beheaded two American journalists and a British aid worker.

The air assault has taken aim at Islamic State checkpoints, training grounds, oil fields, vehicles and bases as well as buildings used as headquarters and offices.

Activists say the militants have cut back the number of gunmen manning checkpoints, apparently fearing more strikes. There has also been an exodus of civilians from Islamic State strongholds.

"Everywhere where there are ISIS buildings, the people living around these buildings are leaving. They are moving far from ISIS buildings, either to other villages or to other areas in the same cities," said Abdurrahman, using an alternative name for the group. "This has happened in Raqqa, in Deir el-Zour and in many towns and villages."

Raqqa, an ancient city located on the Euphrates River in northeastern Syria, is the de facto capital of the Islamic State group's self-declared caliphate.

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