BEIRUT — U.S.-led airstrikes targeted Syrian oil installations held by the militant Islamic State group overnight and early Thursday, killing nearly 20 people as the militants released dozens of detainees in their de facto capital, fearing further raids, activists said.

The latest strikes came on the third day of a U.S.-led air campaign aimed at rolling back the Islamic State group in Syria, and appeared to be aimed at one of the militants' main revenue streams. The U.S. has been conducting air raids against the group in neighboring Iraq for more than a month.

The Islamic State group captured most of Syria's largest oil fields earlier this year and is believed to be partly funding its operations by smuggling oil out and selling it on the black market.

At least four oil installations and three oil fields were hit around the town of Mayadeen in the eastern province of Deir el-Zour, according to the Britain-based Syrian Observatory for Human Rights and two local activist groups. A third activist group loyal to the militants confirmed the reports.

At least 14 militants were killed, said the Observatory, which gathers information from a network of activists on the ground. Another five people who lived near one of the refineries in the northeastern Hassakeh province were also killed, the Observatory said, adding that they were likely the women and children of the militants.

Two activist collectives reported higher death tolls. It is common to have conflicting casualty figures in the chaotic aftermath of such events.

The planes came "with a terrifying sound and red lights before the explosions," one activist wrote, documenting each explosion.

The militants meanwhile freed at least 150 people from a prison in their de facto capital of Raqqa in northeastern Syria, fearing more strikes, according to activists there.

Other strikes hit checkpoints, compounds, training grounds and vehicles of the Islamic State throughout the territory they hold along the Euphrates River in northern and eastern Syria, with strikes hitting near the Turkish and Iraqi borders.

The raids targeted Syrian military bases seized by the Islamic State group, including the Brigade 93 and Tabqa bases. They also hit a building used as an Islamic court and a cultural center in the town of Mayadeen, the activists reported.

The Observatory said other airstrikes targeted the Nusra Front, a Syrian al-Qaida affiliate that has fought the Islamic State, and which is one of the most powerful groups fighting to overthrow Syrian President Bashar Assad. The strikes against the Nusra Front suggest a wider operation targeting other Syrian militants seen as a potential threat to the United States.

The Observatory also reported airstrikes near a northern Kurdish area that Islamic State militants have been attacking for nearly a week now, causing the flight of over 150,000 people to neighboring Syria. But it was not immediately clear who was conducting the airstrikes southwest of the area known as Kobani, or Ayn Arab.

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