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.
Israel attacks Syrian posts after cross-border fire from Syria
Meanwhile, farmers and civilians were ordered to stay away from the Valley of Tears, New Quneitra, and open areas. The projectiles were spillover from the fighting near Syria's disputed border with Israel, the army said.
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.