Syria has used sarin nerve gas for the first time since 2013, dropping bombs laden with the chemical agent on Isis (Islamic State) fighters outside Damascus, according to a senior Israeli official.
This use of sarin would show that President Bashar al-Assad has retained the ability to gas his enemies, despite an agreement that supposedly disarmed Syria of its chemical arsenal.
That deal was reached after the regime used sarin and VX gas to kill as many as 1400 people in rebel-held areas of Damascus on August 21, 2013.
United States President Barack Obama had declared the use of chemical weapons to be a "red line" that would trigger airstrikes.
Once Assad agreed to disarm, however, Obama abandoned his plan for military action. Since then, Assad's forces are believed to have used relatively unsophisticated chlorine gas on several dozen occasions.
But the regime refrained from employing sarin - a far more lethal substance - until the latest incident.
The Syrian air force dropped the bombs laden with sarin just over three weeks ago, said the official. The apparent aim was to prevent Isis from seizing two air bases located north-east of Damascus.
Israel believes that scores of Isis 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.
"They deceived and they still have it [sarin]," the official said. "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 1300 tonnes of mustard, VX and sarin gas. Israel believes that Assad probably disposed of his mustard and VX but 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).
Even before the latest report of Assad's use of sarin, Ahmet Uzumcu, the OPCW's director general, had questioned whether Syria was genuinely disarmed in full.
"We are not yet there," he said on May 1. "There are still questions. I am not able to say whether Syria has declared everything or whether Syria continues to possess some chemical weapons or some munitions." Telegraph Group Ltd