Israel believes that scores of Isil fighters were killed in the incident, although the exact impact is uncertain. The casualties were far heavier than those that would have been inflicted by chlorine gas, leading Israeli analysts to conclude that sarin was used.

After the agreement to disarm Syria, John Kerry, the US secretary of state, claimed: “We got 100 per cent of the chemical weapons out.”

But the Israeli official said that Assad had concealed a significant proportion of his chemical arsenal. In particular, he kept some of his sarin gas.

“They deceived and they still have it [sarin],” said the official. “Recently, they have decided to use it again. Once a taboo is broken, it becomes a standard weapon that you use. There are no red lines and it [sarin] becomes a standard kind of weapon.”

Syria’s chemical arsenal was believed to include 1,300 tons of mustard, VX and sarin gas. Israel believes that Assad probably disposed of his mustard and VX, in accordance with the deal. But he chose to keep the sarin because this was the deadliest chemical weapon at his disposal.

Sarin, which is odourless and invisible, causes convulsions, paralysis and death within minutes. The disarmament of Syria was overseen in 2013-14 by experts from the Organisation for the Prohibition of Chemical Weapons (OPCW).