Syrian ground troops have been battling to take back the last urban area held by "Islamic State" (IS). Airstrikes and artillery have reportedly killed dozens.
Scores of Syrian civilians at two displacement camps have been killed by artillery fire, an independent monitoring agent reported Sunday.
The British-based Syrian Observatory for Human Rights said the attack began Friday night in the Deir el-Zour region and has killed 53 civilians, including children. The latest casualty figures nearly double the original report of 26 dead on Saturday.
The attacks targeted an area along the Euphrates River that includes villages and displacement camps full of people from the border town of Albu Kamal.
Meanwhile, airstrikes on Sunday along two river crossings over 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 5 kilometers east of Albu Kamal," Abdel Rahman told AFP news agency. "Six civilians were killed in raids on another river crossing 20 km (12 miles) north of the town."
Syrian forces loyal to President Bashar al-Assad and backed by Russia captured Albu Kamal from IS militants on Thursday, only to lose the territory to IS again on Saturday.
Without acknowledging the military setback, the Syrian government accused a US-led coalition of hampering Syrian forces trying to fight IS militants.
Syria's foreign ministry also accused the US coalition of committing "crimes" against civilians in the war-torn country, but it did not elaborate. It called on the UN Security Council to take immediate action to "stop monstrous crimes by this illegal alliance."
The last IS city in Syria
The battle for Albu Kamal is strategically important as it is the last town held by IS militants inside Syria. Losing it would force IS to revert to an underground guerrilla organization with no urban base.
IS militants attempted to recement their control of the city on Monday, ambushing Iranian-backed militias from tunnels in the center of the city.
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.
So far, the conflict in Syria has claimed the lives of more than 330,000 people and forced millions more to flee as large swathes of the country have been reduced to rubble.
aw, bik/amp (AFP, dpa, Reuters)