Separate Russian and US-led coalition air strikes on Tuesday killed 35 civilians in Syria's eastern Deir Ezzor province, where parallel offensives against the Islamic State group are under way, a monitor said.

The province lies along Syria's border with Iraq and is seen as a strategic prize by both the Russian-backed Syrian regime and an alliance of Kurdish and Arab forces supported by the US-led coalition fighting IS.

The Syrian Observatory for Human Rights, a Britain-based monitor, had earlier reported a toll of 28 dead in Russian and US-led strikes, but raised the figure after new Russian raids on the village of Hawayej Thiab in the west of the province.

Earlier, the monitor reported Russian air strikes hit a cluster of tents on the western banks of the Euphrates river, killing "16 civilians including five children," according to the Observatory's director Rami Abdel Rahman. 

He said civilians had set up the camp on the edge of the Zaghir Shamiyah village after fleeing their homes in fear of clashes.

Syrian government troops are fighting to the west of the Euphrates River that slices diagonally across Deir Ezzor province, in an assault backed by Moscow.

Separately, the Syrian Democratic Forces, a Kurdish-Arab alliance backed by the US-led coalition, is battling IS east of the river.

The Observatory said deadly US-led coalition raids in support of that operation hit the village of Al-Shahabat earlier Tuesday, killing 12 members of a single family including five children.

IS-held Al-Shahabat lies on the eastern side of the Euphrates, around seven kilometres (four miles) from the river.

The strikes follow three days of suspected Russian raids in Deir Ezzor province that have killed dozens of civilians, according to the Observatory.