At least 26 civilians have died in artillery fire and Russian air strikes on two displacement camps and surrounding villages in eastern Syria, a monitor said on Saturday.
The Syrian Observatory for Human Rights said nine children were among those killed. The villages and displacement camps were full of people who had fled fighting in the town of Albu Kamal.
Syrian regime forces and allied militia took Albu Kamal from the Islamic State (IS) group two days ago, but militants mounted a fierce fightback and recaptured it on Saturday.
Rami Abdel Rahman, who heads the Britain-based Observatory, said artillery fire by pro-regime forces and air strikes by Russian warplanes were pummelling remaining IS-controlled areas in eastern Syria.
“Fifteen civilians including seven children died in a displacement camp near Al-Sukkariyah, west of Albu Kamal, in bombing since Friday night,” said Abdel Rahman.
Another 11 civilians, including two children, had died in the bombardment on a string of villages and a second displacement camp about 50 kilometres northwest of Albu Kamal, he told AFP.
The Observatory relies on a network of sources in Syria and identifies whose planes carry out raids according to type, location, flight patterns and munitions used.
The media collective DeirEzzor24 also reported the deaths, saying entire families had been killed in the bombardment near Al-Sukkariyah.
“Most residents have fled Albu Kamal to nearby villages, and there are very few still on the outskirts of the town,” said the collective's head Omar Abu Layla.
“There are no civilians left inside Albu Kamal,” he told AFP.
Albu Kamal is the last significant Syrian town under IS' control. 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.