The report comes after Syrian rebels killed at least 14 people, among them women, in the village of Khatab in the central province of Hama overnight, state media and the Observatory said.
"At least 20 members of IS were killed and others were injured in air strikes... Targeting an IS training base in Raqa," the Observatory said.
The strikes also destroyed 14 IS military vehicles, the Observatory added.
On June 29, IS declared the establishment of a "caliphate," referring to an Islamic system of rule that was abolished nearly 100 years ago.
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While some rebels initially welcomed the IS -- then known as the Islamic State of Iraq and the Levant (ISIL) -- as a potential ally, its abuses and quest for control turned them against it.
Rebels have been fighting IS since January.
For its part, Syrian President Bashar al-Assad's regime has escalated its attacks against IS positions since the group launched a Sunni militant offensive in neighbouring Iraq a month ago.
Syrian Kurds have been fighting IS since 2013.
In the northern province of Aleppo, IS seized three Kurdish areas to the east of Ain al-Arab (Kobani in Kurdish) after two days of fighting that killed at least 22 jihadists and 18 Kurdish fighters, said the Observatory.
The reports come hours after Syrian state television said rebels carried out a "massacre" that included women and children, while the Observatory said seven men and seven women had been "executed" by rebel fighters.
The Observatory said the rebels accused the residents of Khatab village of "collaboration with the criminal regime", and executed the 14.
Aleppo has been under a massive aerial offensive since December.