At least 16 civilians including several children were killed in air strikes believed to have been carried out by Russian jets Thursday in Syria's Deir Ezzor province, a monitor said.
The eastern province is partly controlled by the Islamic State group, which is under pressure from several fronts in war-torn Syria.
"The civilians were killed as they tried to cross the Euphrates river near the town of Abu Kamal," the Britain-based Syrian Observatory for Human Rights said.
The Observatory -- which relies on a network of sources in Syria and identifies whose planes carry out raids according to type, location, flight patterns and munitions used -- said Russian jets had carried out the strikes.
Abu Kamal is one of the few remaining urban strongholds of IS in Syria, which lost control this week of Raqa to the west, the capital of its so-called "caliphate" the jihadists claimed in 2014.
Russia began an intervention in Syria in support of ally President Bashar al-Assad in 2015, and has helped the regime win back large parts of the country.
More than 330,000 people have been killed in Syria since the conflict erupted in March 2011 with anti-government protests.