File Photo: Fighters from the Free Syrian Army are deployed on a checkpoint in the area of Kafr Jannah on the outskirts of the Syrian town of Afrin on October 19, 2022. AFP
The source, quoted by SANA news agency, credited the army and local groups with the operation that led to the death of IS chief Abu Hasan al-Hashimi al-Qurashi.
IS on Wednesday said he died in battle and announced a replacement to head up its remaining sleeper cells.
The US military's Central Command (CENTCOM) said Hashimi was killed in an operation carried out by Syrian rebels in Daraa province in the country's south in mid-October but said the US provided no support.
In mid-October, Damascus said it had launched a joint operation against IS with former rebels in the province.
At the time, SANA identified one of the slain extremists as Abu Abdel Rahman al-Iraqi.
The security source told the agency that Hashimi "is the same person known as Abdel Rahman al-Iraqi".
He was "killed during a security operation" against IS carried out by "the Syrian army with local groups" in the city of Jassem on October 15, the security source said.
Daraa province was the cradle of Syria's 2011 uprising but it returned to regime control in 2018 under a ceasefire deal backed by Russia, which supports the government. The rebels were allowed to keep light weapons.
The province has seen years of security chaos, including killings and clashes, and IS jihadists have also claimed attacks there.
A rebel fighter who took part in the operation had told AFP there was "an exchange of information" between rebels and the regime to "identify the houses where the jihadists were hiding".
"Nobody told us that the Daesh (IS) chief was among them," the fighter had said.
Abu Abdel Rahman al-Iraqi was among the jihadists killed in the fighting, he added.
Rami Abdel Rahman, head of the Syrian Observatory for Human Rights war monitor, said the Iraqi blew himself up in a house where he was dug in after family members left the building.