At least 34 civilians were killed on Sunday when Russian warplanes targeted ferries carrying them across the Euphrates River near Syria's eastern city of Deir Ezzor, a monitor said.
The Britain-based Syrian Observatory for Human Rights monitor initially reported 21 deaths in Russian air strikes but later raised the toll to 34, saying that "more bodies have been found in the river".
Observatory head Rami Abdel Rahman said nine children were among those killed fleeing fighting and that "dozens" of people were wounded in the strikes.
He said the raids targeted "more than 40 ferries" that had left Al-Boulil town southwest of Deir Ezzor city for the eastern shore of the river.
The Observatory relies on a network of sources inside Syria, and says it determines whose planes carry out raids according to type, location, flight patterns and munitions used.
Moscow intervened in Syria in September 2015 in support of its ally President Bashar al-Assad.
Sunday's reported raids came as Syrian troops pressed an offensive against Islamic State group militants across Deir Ezzor province with Russian air cover.
At the same time, an alliance of US-backed Syrian Kurdish and Arab fighters is also on the move against IS in Deir Ezzor.
The Syrian Democratic Forces on Saturday announced they had begun clearing IS jihadists from areas east of the Euphrates, which cuts diagonally across the province.
On Sunday, the Observatory said fighters from the SDF's Deir Ezzor Military Council (DEMC) had made a sweeping advance, capturing territory just kilometres (miles) from the provincial capital Deir Ezzor.