Here's what we know about the operation, including from a fighter, an activist and a war monitor.

- Death of IS leader -

IS said Wednesday that its leader Abu Hasan al-Hashimi al-Qurashi -- an Iraqi national -- was killed "in combat with enemies of God", without elaborating on the date or circumstances of his death.

The US military's Central Command (CENTCOM) said he was killed in a mid-October operation carried out by rebels it referred to as the Free Syrian Army, in restive Daraa province.

CENTCOM confirmed to AFP on Thursday that the operation took place in the city of Jassem, adding that the US provided no support.

- How was the operation organised? -

Syrian government authorities told community leaders in Jassem that IS jihadists were hiding in the city, said opposition activist Omar al-Hariri.

The authorities suggested former rebels mount an attack against the Sunni Muslim extremists, he added.

A fighter who took part in the operation told AFP there was "an exchange of information" between rebels and the regime to "identify the houses where the jihadists were hiding".

"The information available to us indicated that they were Daesh cells, that it was their headquarters for southern Syria and mentioned the leader in charge of Daraa," the fighter added, requesting anonymity.

"Nobody told us that the Daesh (IS) chief was among them."

- What happened? -

Fighters from Jassem, backed by the Fifth Corps -- a Russian-backed formation that has absorbed many former rebels -- carried out the operation, while Syria regime forces surrounded the city and provided artillery cover, several sources told AFP.

The combat lasted five days, targeted 20 houses and involved around 100 jihadists, the fighter said, adding that two blew themselves up.

An Iraqi national known as Abu Abdel Rahman al-Iraqi was among the jihadists killed in the fighting, he said.

But Rami Abdel Rahman, head of the Syrian Observatory for Human Rights war monitor, said Iraqi blew himself up in a house where he was dug in after family members left the building.

He "might have been" the IS chief, said Abdel Rahman.

Regime officials who came to identify the bodies said Iraqi was the IS "emir" in the Daraa region, the fighter said.

- What's happening in Daraa? -

On October 14, Syrian state news agency SANA said "local groups backed by the army" had launched an operation against jihadists in Jassem, adding a day later that IS "emirs" including an Iraqi were targeted.

It later reported that "all the members of the terrorist group have been eradicated".

Hassan Hassan, an expert on IS, said it was possible Hashimi "was killed 'accidentally' during a raid or fighting without him being known to whoever killed him".

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 backs the government, with rebels 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.

For opposition activist Hariri, "the chaos that reigns in Daraa is the main reason IS cells hid there".

The Damascus government has not commented since Wednesday's IS announcement.