Dozens of civilians have died in artillery fire and Russian bombardment of two displacement camps and surrounding territory in eastern Syria, a monitor said Sunday in a new toll.
The Syrian Observatory for Human Rights said ongoing heavy bombardment in the Deir Ezzor province had killed 50 civilians, including 20 children, since late Friday night.
The new toll was nearly double the Britain-based monitor's count on Saturday of 26 dead.
The bombardment has targeted territory along the Euphrates River, as well as villages and displacement camps full of people fleeing fighting in the Syrian border town of Albu Kamal.
Russian-backed Syrian regime forces and allied militia seized Albu Kamal from the Islamic State group on Thursday but the jihadists retook it late Saturday.
On Sunday, Russian air strikes on two river crossings along the Euphrates killed 11 civilians, according to Observatory chief Rami Abdel Rahman.
"Five civilians, including two children, were killed in the strikes on Al-Soussa crossing about five kilometres east of Albu Kamal," Abdel Rahman told AFP.
"Six civilians were killed in raids on another river crossing 20 kilometres north of the town," he added.
The toll from earlier artillery fire and air strikes on two displacement camps and surrounding villages rose to 39 after 13 civilians succumbed to their wounds, the Observatory said on Sunday.
Albu Kamal is the last significant Syrian town IS controls. Losing it would cap the group's reversion to an underground guerrilla organisation with no urban base.
IS rose to prominence in the chaos of Syria's conflict, which broke out in 2011 with protests against President Bashar al-Assad.
It has since evolved into a complex war that has killed more than 330,000 people, forced millions more to flee, and left much of the country in ruins.