Syria agreed to give up its chemical weapons arsenal to avert USA strikes following a chemical weapons attack in opposition-held suburbs of Damascus in August 2013 that killed hundreds of people and sparked worldwide outrage.
US President Donald Trump characterized the strike as a response to the alleged use of chemical weapons by the Syrian government troops while Russian Foreign Minister Sergei Lavrov said it was a violation of the global law.
Israeli defence officials also alleged that the chemical attack was ordered by Syrian military commanders and Assad was aware of the move.
The results of the analysis reached by the OPCW Fact-Finding Mission concur with those of other analyses conducted in Turkish and British laboratories.
The officials delivered the assessment Wednesday, weeks after a chemical attack in Syria killed at least 90 people.
A draft decision put forward by Moscow and Tehran had called for a new investigation by the Organisation for the Prohibition of Chemical Weapons (OPCW) "to establish whether chemical weapons were used in Khan Sheikhoun and how they were delivered to the site of the reported incident".
A senior military official told reporters that the Israeli intelligence estimates that Assad has "between 1 and 3 tons" of chemical weapons. After the meeting, USA secretary of state Rex Tillerson stated the USA strikes were a matter of national security: "We do not want the regime's uncontrolled stockpile of chemical weapons to fall into the hands of ISIS, or other terrorist groups and want to attack the United States and our allies".
In April, US forces attacked a Syrian air base with cruise missiles in retaliation for a deadly chemical weapons attack on Syrian civilians.
Israel on Wednesday said it believes Syrian regime forces still possess several tonnes of chemical weapons.
U.S. Secretary of Defense Jim Mattis initially said 20 percent of Syria's air force had been destroyed in the cruise missile attack, but later backtracked and said that 20 aircraft had been rendered inoperable.
It was reported at the time that the entire stockpile was dismantled and shipped out of the country under worldwide supervision in 2014, and was later destroyed.
In 2013, Assad had signed an agreement with Russia, US to destroy all its chemical weapons.
He says further results will follow but that "the analytical results already obtained are incontrovertible". Israel welcomed the strike on its northern neighbor.
As for the impact of that incident, the adviser said that the recent United States attack on Sheyrat's Syrian airbase in the province of Homs had a political rather than a military goal, insisting that Syria will respond to this "aggression" in the appropriate time.