Syrian president Bashar Assad still has up to three tons of chemical weapons, Israeli defence officials said.
They delivered the assessment weeks after a chemical attack in Syria killed at least 90 people.
Israel, along with much of the international community, believes that Assad’s forces carried out the attack.
Israeli military intelligence estimates that Assad has “between one and three tons” of chemical weapons, a senior military official said.
The assessment was confirmed by two other defence officials.
Assad has denied the allegations that he was behind the April 4 attack.
Syria agreed to give up its chemical weapons arsenal in 2013.
The US and many other nations have called the incident in the opposition-held town of Khan Sheikhun in Syria’s southern Idlib province a chemical weapons attack and accused the Syrian government of responsibility.
In response, the United States fired nearly 60 missiles at a Syrian air base it suspected of being the launching pad for the attack.
Israel, which welcomed the US strike, was notified two hours ahead of time, the military official said.
The Syrian government has been locked in a six-year civil war against an array of opposition forces.
The fighting has killed an estimated 400,000 people and displaced half of Syria’s population.
Assad agreed in 2013 to declare and dispose of all his chemical weapons under UN supervision, but his forces have repeatedly been accused of using them since then.
The disarmament, which was carried out amid a chaotic conflict, has always been the subject of some doubt, and there is evidence that Islamic State and other insurgents have acquired chemical weapons.
A fact-finding mission from the Organisation for the Prohibition of Chemical Weapons, an international watchdog, is investigating the incident and is expected to issue a report within two weeks.
Turkish and British tests have also concluded that sarin or a substance similar to the deadly nerve agent was used in the Idlib attack.
Syria agreed to give up its chemical weapons arsenal to avert US strikes in September 2013, following a chemical weapons attack in the Damascus suburbs in August that year that killed hundreds of people and sparked worldwide outrage.
Ahead of disarmament, Assad’s government disclosed it had some 1,300 tons of chemical weapons, including sarin, VX nerve agent and mustard gas.
The entire stockpile was said to have been dismantled and shipped out under international supervision in 2014 and destroyed.
The chemical weapons were shipped outside Syria and destroyed abroad, with the most toxic material disposed of at sea aboard a US ship.
But doubts began to emerge soon afterwards that not all such armaments or production facilities were declared and destroyed.
Earlier this week, Assad’s former chemical weapons research chief told Britain’s The Telegraph that Syria had “at least 2,000 tons” of chemical weapons before the war and only declared 1,300.
Former Brigadier General Zaher al-Sakat said the Syrian government still possessed hundreds of tons of chemical weapons.
Israel has largely stayed out of the civil war raging in its northern neighbour.
But it has carried out a number of air strikes against suspected arms shipments bound for Assad’s ally Hezbollah in Lebanon, and in retaliation to errant fire into the Golan Heights.