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' rule.
It said the Mayadeen mission was "meticulously planned and executed to reduce the risk.to non-combatants".
Dillon added that Observatory reporting had previously been exaggerated.
Both Deir Ezzor 24 and SOHR identified aircraft from the US-led coalition as the alleged source of the bombing but it was not clear how they had identified the aircraft responsible.
Before turning it into a prison, the building was the home of the leader of al-Qaida affiliate in Syria who was assassinated by Islamic State.Читайте также: Israel attacks Syrian posts after cross-border fire from Syria
The US-backed Syrian Democratic Forces (SDF), an alliance of Kurdish, Arab and other forces, have encircled the de-facto capital of Raqqa with Mosul, around 200km north-east, in Iraq, also set to fall imminently.
Islamic State is believed to have moved most of its leaders to al-Mayadeen in Syria's Euphrates Valley, south-east of the group's besieged capital Raqqa, two USA intelligence officials have said.
In the meantime, a local media activist told ARA News that ISIS is using civilians as human shields in Deir ez-Zor.
"This allegation will be provided to our civilian casualty team for assessment", he said.
The US-led coalition said last week that it had killed IS's top cleric Turki Binali in a May 31 strike on Mayadeen.При любом использовании материалов сайта и дочерних проектов, гиперссылка на обязательна.
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