Senior Israeli defense officials on Wednesday said that the regime of Syrian President Bashar Assad is believed to have as much as three tons of chemical weapons, despite a 2013 agreement ordering the country to dispose of its stockpiles, The Associated Press reported.
French Foreign Minister Jean-Marc Ayrault said French intelligence services would provide proof of that in the coming days.
Sarin or a similar banned toxin was used in an attack in Syria's Idlib province on April 4 that killed almost 90 people, the global chemical weapons watchdog says.
The US responded to the attack by launching air strikes on a Syrian military airfield.
Earlier this week, Assad's former chemical weapons research chief told Britain's The Telegraph newspaper that Syria had "at least 2,000 tons" of chemical weapons before the war and only declared 1,300.
Mr Uzumcu said samples taken from three people who died in the attack were analysed at two OPCW designated laboratories.
After the USA strike, Russian Federation said it would suspend its participation in a hotline with the United States military to "deconflict" or minimize the risk of mid-air collisions in Syrian airspace, but US officials said this week that the two militaries continued to communicate. The chemical weapons were shipped outside Syria and destroyed overseas, with the most toxic material disposed of at sea aboard a USA ship.
In their Final Communiqué on April 10-11, G-7 leaders stated that USA action against the Shayrat airfield "was a carefully calibrated, limited in scope response" to the attacks at Khan Sheikhoun "in order to prevent and deter the proliferation and use of deadly chemical weapons in Syria".
"Even if we have them, we wouldn't use them", Assad said in an exclusive interview with AFP in Damascus on Wednesday.
Dan Kaszeta, a United Kingdom -based chemical weapons expert, said the Israeli estimate appeared to be conservative, but nonetheless was enough to be highly lethal.
"One ton of sarin could easily be used to perpetrate an attack on the scale of the 2013 attack", Kaszeta said.
It also calls for investigators to visit the Shayrat airbase - bombed by the United States after the April 4 attack - to "verify allegations concerning the storage of chemical weapons" there.
Such a move would also be against the convention against chemical weapons "as it is the role of the OPCW to lead independently any investigation", he added.