Airstrikes and artillery have reportedly killed dozens.
At least 50 civilians, including 20 children, were killed on Monday in airstrikes on Syria's northern Aleppo province despite a "de-escalation zone" in place there, a monitor said.
The new toll was almost double the Britain-based monitor's count on Saturday of 26 dead.
"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.
The Observatory said the bombardment has targeted territory along the Euphrates River, as well as villages and displacement camps full of people fleeing fighting in Albu Kamal town.
Meanwhile, airstrikes on Sunday along two river crossings over the Euphrates killed 11 civilians, according to Observatory chief Rami Abdel Rahman.
Trump asked Xi Jinping to help UCLA basketball players busted shoplifting
Several experts in Chinese law said charges and accompanying punishment will dependent on the value of the stolen goods . Xi promised to look into the case and ensure that the players be treated fairly, a USA official told the paper.
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.
Syria's foreign ministry also accused the USA coalition of committing "crimes" against civilians in the war-torn country, but it did not elaborate.
Albu Kamal is the last significant Syrian town IS controls. Losing it would force IS to revert to an underground guerrilla organization with no urban base.
Despite the government's recapture of Aleppo city late previous year, rebel groups maintain a presence in the west of the province.
IS militants had capitalized on the confusion that followed Syria's descent into civil war in 2011 to carve out a large swathe of territory in both Syria and neighboring Iraq.
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