BEIRUT (AP) — 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 strikes aimed to knock out one of the militants’ main revenue streams — black market oil sales that the U.S. says earn up to $2 million a day for the group. That funding, along with a further estimated $1 million a day from other smuggling, theft and extortion, has been crucial in enabling the extremists to overrun much of Syria and neighboring Iraq.

The United States and its Arab allies have been carrying out strikes in Syria for the past three days, trying to uproot the group, which has carved out a self-declared state straddling the border, imposed a harsh version of Islamic law and massacred opponents. 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 raged on unabated, with government forces taking back an important industrial area near Damascus from the rebels, according to Syrian activists and state media. Activists also accused 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. The new strikes involved six U.S. warplanes and 10 more from the United Arab Emirates and Saudi Arabia, mainly hitting small-scale refineries used by the militants in eastern Syria, Pentagon spokesman Rear Adm. John Kirby said.

At least 14 militants were killed, according to the Britain-based Syrian Observatory for Human Rights, which monitors the Syrian conflict through a network of activists on the ground.

The Observatory and two independent activists said another five people who lived near one of the refineries were also killed, likely the wives and children of the militants.

Kirby said the Pentagon is looking into reports that civilians were killed but has no evidence yet.

Other strikes hit checkpoints, compounds, training grounds and vehicles of the Islamic State group in northern and eastern Syria. The raids also targeted two Syrian military bases that had been seized by the Islamic State group. In the eastern Syrian 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. Many of the casualties the group has sustained in the American-led air raids have been at checkpoints. Activists also said that more families of Islamic State militants were clearing out of the city of Raqqa, the group’s de facto capital, on Thursday, heading eastward.

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.

Some opposition activists saw the strikes on the Nusra Front as a sign of a wider operation targeting other Syrian militants among the anti-Assad rebellion seen as a potential threat by the United States.

“All of this is to serve Bashar, 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 of 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 fewer 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 more than 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 Ayn Arab, or Kobani to the Kurds. 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 the rebel-held industrial area of Adra near Damascus after months of clashes.

On a government-organized tour of the area Thursday, the smell of dead bodies hung in the air amid the bombed-out buildings and torched cars. An unnamed commander accompanying the journalists said that the military dismantled 17 car bombs, and that soldiers were working to disarm more of them.

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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Associated Press writer Lolita C. Baldor in Washington and Albert Aji in Adra al-Omaliya, Syria, contributed to this report.