Assad has denied any involvement with the chemical weapons attack that occurred in Khan Sheikhun April 4, killing 87 people. All the officials spoke on condition of anonymity under military briefing rules.
The state of the Syrian government's chemical weapons program has been the subject of fierce scrutiny in recent weeks, after a deadly chemical attack in northern Syria earlier this month killed more than 80 civilians.
The finding supported earlier testing by Turkish and British laboratories.
Senior Israeli defence officials believe Assad could possess up to three tons of chemical weapons, it was reported today. Top Assad ally, Russia, has asserted a Syrian government airstrike hit a rebel chemical weapons factory, causing the disaster.
The Foreign Secretary said there was "only one conclusion, that the Assad regime nearly certainly gassed its own people in breach of worldwide law and the rules of war", showing disregard for an agreement to destroy its stockpiles in 2013.
The missile strike was the first direct USA military action against Assad's forces since the start of Syria's civil war six years ago and precipitated a downward spiral in ties between Washington and Moscow.
But questioned remained about the efficacy of the agreement and whether the regime had disposed of all its chemical weapons.
Theresa May welcomed the air strikes, as did European allies including Germany and France, but the Conservative Party is reportedly divided over the extent of British involvement in any future operations against Assad. But doubts began to emerge soon afterward that not all such armaments or production facilities were declared and destroyed.
Former Brig. Gen. Zaher al-Sakat said the Syrian government still possessed hundreds of tons of chemical weapons. If you would like to discuss another topic, look for a relevant article.
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