The Britain-based Syrian Observatory for Human Rights said the toll is based on information from sources in Syrian military hospitals.
The Syrian government has not released a death toll. Immediately after Sunday’s pre-dawn strike, Syrian state media said the attack near Damascus caused casualties, but did not elaborate.
So far, Israel has carried out three airstrikes in Syria this year, according to Israeli and US officials, although Israel’s government has not formally confirmed involvement.
The officials say the attacks were meant to prevent advanced Iranian weapons from reaching Lebanon’s Hezbollah militia, a Syrian ally and Israel foe.
Israel earlier signalled a return to “business as usual,” with Prime Minister Benjamin Netanyahu arriving in China for a scheduled visit,
Syria and its patron Iran have hinted at possible retribution over the strikes, although the rhetoric in official statements has been relatively muted.
Iranian Foreign Minister Ali Akbar Salehi warned that Israel was “playing with fire”, but gave no other suggestions of possible consequences, according to the official Islamic Republic News Agency.
Syria’s government called the attacks a “flagrant violation of international law” that has made the Middle East “more dangerous”.
Meanwhile, a UN panel looking into war crimes in Syria said it has not found conclusive evidence of chemical weapons use, backing away from a member’s claims that there are indications rebel forces used the nerve agent sarin.
The commission “wishes to clarify that it has not reached conclusive findings as to the use of chemical weapons in Syria by any parties to the conflict,” the panel said.
The statement comes after panel member and former war crimes prosecutor Carla Del Ponte told Swiss TV that the commission has indications that Syrian rebel forces used nerve agent sarin as a weapon but not yet incontrovertible proof.