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Local sources said a roadside bomb exploded and killed Esber and his driver in ​​the area, which is located 100 kilometers north of Damascus. 

Official sources told Prensa Latina agency that Israeli intelligence services may be involved. Israel has been blamed for a strike on Esber's centre in September 2017.

Britain-based war monitor, the Syrian Observatory for Human Rights, at the time said the strikes hit the Scientific Studies and Research Centre facility, the agency the US describes as Syria's chemical weapons manufacturer.

No official comment from the Syrian authorities has been made.

There have been  across Syria since 2013, the vast majority of which were carried out by the regime, according to Human Rights Watch. The Independent International Commission of Inquiry on the Syrian Arab Republic .

After hundreds of people were killed in chemical attacks near Damascus in August 2013, a landmark deal with Russia was struck to rid Syria of its chemical weapons stash, staving off US airstrikes.

Despite the deal, last month the Organisation for the Prohibition of Chemical Weapons said that its fact-finding mission in Syia found "sarin was very likely used as a chemical weapon in the south of Ltamenah" in Hama province on 24 March and that chlorine was very likely used a day later at and near Ltamenah Hospital.

And in April a suspected chlorine and sarin attack in the Syrian town of Douma on 7 April left at least 49 people dead.