US-led air strikes have targeted Syrian oil installations held by the Islamic State group, killing nearly 20 people, as the militants released dozens of detainees in their de facto capital, activists said.
The latest strikes came on the third day of an air campaign aimed at pushing back the IS group in Syria, and appeared to be aimed at one of the militants' main revenue streams.
IS is believed to control 11 oil fields in Iraq and Syria, and to earn more than 3 million US dollars (£1.8 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-styled caliphate 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 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 north-eastern Hassakeh province were also killed, the Observatory said, adding that they were probably wives and children of the militants.
The militants freed at least 150 people from a prison in their the city of Raqqa in north-eastern Syria, fearing more strikes, according to activists there.
Other strikes hit checkpoints, compounds, training grounds and vehicles throughout the territory IS holds along the Euphrates River in northern and eastern Syria, with strikes hitting near the Turkish and Iraqi borders.
The US has been conducting air raids against the group in neighbouring Iraq for more than a month.
The raids targeted Syrian military bases seized by IS, including the Brigade 93 and Tabqa bases. They also hit a building used as an Islamic court and a cultural centre in the town of Mayadeen, the activists reported.
The Observatory said other air strikes targeted the Nusra Front, a Syrian al Qaida affiliate which has battled IS and 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 US.
The Observatory also reported air strikes near a northern Kurdish area that IS militants have been attacking for nearly a week, causing the flight of more than 150,000 people to neighbouring Turkey. It was not immediately clear who was conducting the air strikes south west of the area known as Kobani, or Ayn Arab.
A senior Kurdish fighter, Ismet Sheikh Hasan, said there were three air strikes on the outskirts of Kobani overnight, but fighters were not able to approach the area to see the target of the strikes.
Elsewhere in Syria, forces loyal to Assad wrested back a rebel-held industrial area near Damascus after months of clashes, according to the Observatory and pro-Assad Lebanese media.
The pro-government forces seized the Adra industrial zone after rebels accused them of using chemical explosives.