The Syrian Observatory for Human Rights said the bombing raids hit the village of Al-Jerzi on the eastern bank of the river, which cuts across Deir Ezzor province.
"Twenty-one civilians were killed, including nine children, very early Wednesday in Russian air strikes targeting residential buildings in Al-Jerzi," Observatory head Rami Abdel Rahman told AFP.
The Britain-based monitor 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.
It says IS, which used to control swathes of Deir Ezzor province, has been ousted from all but eight percent of the oil-rich region.
The jihadists have lost vast swathes of it to separate offensives by Russian-backed Syrian troops and an alliance known as the Syrian Democratic Forces.
The SDF has long been backed by US-led coalition bombing of IS in Iraq and Syria, but its Kurdish component recently said it had also received support from Moscow.
Russian warplanes had given air cover to the Kurdish People's Protection Units (YPG) as they fought against jihadists in Deir Ezzor, according to the YPG and Moscow's defence ministry.
Russia first launched bombing raids in 2015 in support of Syrian President Bashar al-Assad's beleaguered forces.
Those strikes have helped Assad regain control over much of war-ravaged Syria.
More than 340,000 people have been killed since the conflict broke out with protests against Assad's rule in March 2011 which were met with a brutal crackdown.
IS has also lost most of the territory it held in neighbouring Iraq.
