ISIS has lost a string of its high-ranking commanders in recent weeks to strikes by the US-led coalition which launched an air campaign against the jihadists in Iraq and Syria in 2014, AFP reports.
A drone strike on Wednesday, likely by the coalition, killed Abu al-Haija, a Tunisian commander summoned by IS leader Abu Bakr al-Baghdadi from Iraq.
Fifteen ISIS commanders accused of revealing Abu al-Haija's position have since been executed by the jihadist group, the Syrian Observatory for Human Rights said Sunday.
The fate of another 20 men accused of collaborating with the US-led coalition remains unknown, it added.
"This is the highest number of executions of security officials by ISIS," Abdel Rahman said.