Separate Russian and US-led coalition air strikes on Tuesday killed 28 civilians in Syria's eastern Deir Ezzor province, where rival offensives against the Islamic State group are under way, a monitor said.

"Coalition air strikes killed 12 members of a single family, among them five children, in a village on the eastern banks of the Euphrates River," said Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights.

Russian air strikes in support of a separate Syrian government offensive against IS killed 16 civilians, including five children, northwest of Deir Ezzor city, he said.