Local sources said a roadside bomb exploded and killed Esber and his driver in the area, which is located 100 kilometers north of Damascus.
There have been 85 chemical attacks across Syria since 2013, the vast majority of which were carried out by the regime, according to Human Rights Watch.
An insurgent group calling itself the Abu Amara Brigades claimed responsibility for the operation.
Syrian officials accused Israeli intelligence of carrying out the assassination.
A senior Syrian scientist involved in the Assad regime's chemical weapons programme has been assassinated in central Syria, reports claimed on Saturday night.
The research centre that Asber led in the western city of Masyaf has previously been targeted by air strikes.
Missile strikes by the US, Britain and France destroyed another Syrian scientific studies and research centre facility in Damascus in April, in response to a suspected poison gas attack.
The U.S. leveled sanctions against 271 employees of the SSRC less than three weeks after the attack, saying the agency was responsible for "developing and producing non-conventional weapons and the means to deliver them". As reported TASS, the center has twice been bombed by the Israeli air force, which operated out of Lebanese airspace.
The activist-run Suwayda 24 Facebook group said 19-year-old Muhanad Thouqan Abou Ammar was killed by the militants on Thursday.
The Observatory said Abou Ammar was the first Sweida hostage killed by the group.
More than 350,000 people have been killed and millions displaced since the start of Syria's civil war in 2011.
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