Syrian President Bashar al Assad could still possess up to three tons of chemical weapons, senior Israeli defense officials say, AP reported. The Syrian government was supposed to completely give up its chemical weapons arsenal under an agreement with the United States and Russia in 2013. The deal came after government forces used chemical weapons against rebel-held areas outside of Damascus, killing more than 1,400 people. That agreement was supposed to rule out the use of chemical weapons in Syria, even as the country became more embroiled in a complex civil war. The U.S. government and its allies say Damascus used chemical weapons again in early April, striking a rebel-held town in Idlib province with sarin or a similar type of nerve gas. Syria and Russia deny government forces were responsible for the attack, which killed nearly 90 people and triggered a U.S. missile strike in response. The warnings by top Israeli defense officials that Damascus could still possess large quantities of chemical weapons come as the Organization for the Prohibition of Chemical Weapons said test results following the attack earlier this month show incontrovertible use of sarin or a similar substance.