U.S.-led airstrikes targeted Syrian oil installations held by the extremist Islamic State group overnight and early Thursday, killing at least 19 people as more families of militants left their key stronghold, 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.

On the ground, Syria's civil war continued unabated, with government forces taking back an important industrial area near Damascus from the rebels, said Syrian activists while also accusing President Bashar Assad's troops of using an unspecified deadly chemical substance.

The Islamic State group is believed to control 11 oil fields in Iraq and Syria, earning more than $3 million a day from oil smuggling, theft and extortion.

Those funds have supported its rapid advance across much of Syria and Iraq, where it has carved out a self-declared state straddling the border, imposed a harsh version of Islamic law and massacred opponents.

At least four oil installations and three oil fields were hit around the town of Mayadeen in eastern Syria, according to the Britain-based Syrian Observatory for Human Rights and two local activist groups. It wasn't immediately clear how important the refineries and fields were.

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 were also killed, likely the wives and children of the militants, the Observatory and activists said.

Other strikes hit checkpoints, compounds, training grounds and vehicles of the Islamic State in northern and eastern Syria.

The raids also targeted two Syrian military bases that had been seized by the Islamic State group. In the town of Mayadeen, a building used by the militants as an Islamic court was also hit.

Apparently fearing more strikes, the militants reduced the number of fighters on their checkpoints, activists said. More families of Islamic State militants left Raqqa city, the group's de facto capital, heading eastward, they added.

For some Syrians, the airstrikes were bitter justice.

"God has imposed on you just a part of what you have done, but you are even more criminal," wrote Mahmoud Abdul-Razak on an anti-Islamic State group Facebook page, saying that the airstrikes were divine punishment.

But other Syrians see coalition strikes as serving Assad's interests because they do not target government forces and because some have hit the Nusra Front, Syria's al-Qaida affiliate that has battled both the Islamic State and Assad's forces

The strikes against the Nusra Front suggest a wider operation targeting other Syrian militants seen as a potential threat to the United States.

"All of this is to serve Bashar (Assad), and yet people believe the Americans are protecting the Syrians," said Saad Saad, writing on the same Facebook page.

A rebel fighter in the northern Aleppo province who only identified himself by his nom de guerre, Ramy, said the U.S. airstrikes appear coordinated with the flights Syrian military planes, which would disappear from the skies shortly before the U.S.-led coalition aircraft show up.

"It's like they coordinate with each other," Ramy told The Associated Press over Skype. "The American planes come and they go."

The Observatory reported less Syrian airstrikes in the past three days — likely because of the presence of the coalition aircraft. Still, bombing continued in a rebel-held area near Damascus, killing at least 8 people, including children, reported the Observatory and activist Hassan Taqulden.

Syrian Kurdish fighters also reported three airstrikes near a northern Kurdish area, which Islamic State militants have been attacking for nearly a week, prompting over 150,000 people to flee to neighboring Turkey.

The Kurdish fighters said the U.S.-led coalition was likely behind the strikes in the area known as Kobani, or Ayn Arab. A spokesman for the fighters, Reydour Khalil, pleaded again that the coalition coordinate with them, claiming that the overnight strikes were not effective and struck abandoned bases

"We are willing to cooperate with the U.S. and its alliance" by providing positions and information about the militants' movements, Khalil said.

Elsewhere in Syria, Assad's forces wrested back a rebel-held industrial area near Damascus after months of clashes, the Observatory and pro-Assad media in Lebanon said.

The government forces seized the Adra industrial zone after rebels accused them of using chemical explosives there on Wednesday. Footage of the wounded from the incident, in which six people were killed, showed men jerking uncontrollably and struggling to breathe before their bodies went limp.

The footage, posted on social networks, appeared genuine and consistent with The Associated Press reporting of the event depicted.

But the footage did not suggest what chemical — if any — was used on the men.

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