The UK-based war monitor said the air strike took place on Monday at dawn, hitting a building in al-Mayadeen used to house dissenters against Isis's rule. At least 15 Isis fighters and jailers were reported among the dead in the attack in Deir Ezzour province.
Activist-run media outlet Deir Ezzour 24 put the toll at 60 civilians, adding that the building belonged to an al-Qaeda-linked commander before it was seized by the Isis in 2014.
It was not immediately clear how either the Observatory or Deir Ezzour 24 identified the aircraft responsible for the strike. Syrian state television also said the US-led coalition had carried out the bombing and that the building was known to hold a "large number" of civilians.
The Russian and Syrian air forces also carry out frequent attacks on Isis positions across the country.
A spokesperson for the coalition has said it is investigating the reports. "With every single allegation we will take it and look into it ... If we are responsible for any civilian casualties we come forth and admit it," US Colonel Ryan Dillon told Reuters, adding that previous Observatory reporting was exaggerated.
Independent monitor AirWars says the civilian death toll since the US-led bombing campaign against Isis across Syria and Iraq began in 2014 totals at least 3,800 people. The Observatory's estimate is 1,481 dead, among them 319 children.
The coalition itself has admitted culpability for the deaths of 484 civilians. Since US President Donald Trump entered office in January this year, delegating more authority to his generals, there has been a marked uptick in civilian deaths in bombing operations in the two countries.
Many of Isis's leaders are thought to be in hiding in the area around al-Mayadeen as the extremists continue to lose ground; the organisation now holds onto just a fraction of the territory under its control at the height of its powers in 2014. Twin US-backed campaigns to oust fighters from their last urban strongholds – Raqqa in Syria and Mosul in Iraq – are under way, led by local forces on the ground assisted by 5,000 US military advisors.
The complex Syrian civil war has killed almost 500,000 people, the UN says, and is now in its seventh year.
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