Russian and Syrian government airplanes killed at least 45 people, including children, Saturday in a town in eastern Syria, the Britain-based Syrian Observatory for Human Rights said.
The monitoring group said that the strikes hit the town of Qourieh in Deir el-Zour province, which borders Iraq and is mostly controlled by the Islamic State militant group.
Meanwhile, the Syria Democratic Forces (SDF) rebel group, a coalition of Kurdish and Arab forces backed by aerial support from the U.S.-led coalition, clashed with IS inside Manbij, a key stronghold of the extremist group.
The SDF pushed into the town from the southern edge, after gaining control of a wheat silo complex on its outskirts.
Manbij had been one of the major supply stops for IS militants through areas of northern Syria they control. Local activists say loss of the town would be a significant setback for the terror group.
Hundreds of Kurds have come under fire in northern Syria, several of them killed and wounded while trying to flee the crossfire of competing forces.
The exodus began after IS abducted about 900 Kurdish civilians in Aleppo province over the past three weeks. Others were trying to flee Manbij surrounded by SDF fighters backed by the U.S.-led coalition.
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 and since then has blown into a multi-front war that has drawn in global powers.