(AP) -- 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 generate 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.

There was no immediate confirmation from the United States or 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 group reported another apparent coalition air raid on Islamic State positions outside the city of Hassakeh in northeastern Syria near the Iraqi border. Those strikes targeted an 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.

The coalition campaign expands upon the airstrikes the U.S. has been conducting on its own against the extremist faction for more than a month in Iraq. France joined the American effort in Iraq a week ago, and is considering whether to extend its airstrikes to Syria as well.

Denmark, meanwhile, announced Friday it would send seven F-16 fighter jets to take part in the airstrikes in Iraq, two days after the Netherlands did so. Britain and Belgium are also debating their involvement in the coalition Friday. The European countries do not plan to deploy in Syria.

The international operation targeting the Islamic State group adds another layer to Syria's civil war, a conflict that has already killed more than 190,000 people since the revolt against President Bashar Assad began in March 2011.

While overshadowed by the coalition strikes against the Islamic State, fighting between Syrian government troops and rebels has raged on with its usual ferocity.

Assad's warplanes struck opposition-held towns in several provinces, including Hama in central Syria and Daraa in the south, activists said. There was no immediate word on casualties.

In Damascus, government soldiers backed by gunmen from the Lebanese Shiite militant Hezbollah group battled rebels on the edges of the suburb of Jobar, the Observatory said.

The Islamic State group meanwhile continued to press its offensive near the Turkish border against Syrian Kurds, closing in on the border town of Ayn Arab, also known as Kobani. The militants have overrun dozens of villages the area in recent weeks as they look to clear out one of the few remaining pockets of resistance to their rule in northern Syria.

A senior Kurdish fighter overseeing the defense of Kobani, Ismet Sheikh Hassa, said that Islamic State fighters were advancing on the city from three sides Friday and were now launching mortars and rockets into Kobani itself.

He said that Kurdish fighters were outgunned, using old Russian weapons and assault rifles to fend off the attack.

"They have heavy weapons," including tanks, artillery and machine-guns, Hassa said by telephone. "The Islamic State is firing mortars and rockets over Kobani randomly. There are numerous civilian causalities."

He said that coalition airstrikes had been mostly directed against targets in other parts of the country and had done little to ease the siege of Kobani.

Also on Friday, scores of Kurdish activists and fighters were seen removing barbed wire and crossing the Turkish border to help defend the city.

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Associated Press writer Maamoun Youssef in Cairo, Desmond Butler in Sanliurfa Turkey, and Burhan Ozbilici contributed to this report.