Russian and regime air strikes killed 47 people, most of them civilians, and wounded dozens more on Saturday in a jihadist-held town in eastern Syria, a monitor said.
The Syrian Observatory for Human Rights said the raids hit the town of Al-Quriyah, controlled by the Islamic State group in Syria's oil-rich Deir Ezzor province.
Observatory head Rami Abdel Rahman said 31 civilians were identified among those killed, but that it was not immediately clear whether the 16 others were civilians or IS fighters.
"IS fighters have now set up a security perimeter around the residential area, where the town's mosque is located," said Abdel Rahman.
The Britain-based monitor -- which relies on a network of sources inside Syria for its information -- says it determines what planes carried out raids according to their type, location, flight patterns and the munitions involved.
Russian warplanes have been carrying out an air war in support of President Bashar al-Assad since September 2015.
More than 280,000 people have been killed since Syria's conflict erupted in March 2011, after a widespread protest movement evolved into a complex, multi-front war that has drawn in global powers.