It's a nightmare scenario that is being taken seriously by western governments as the Syrian end game nears.
What if a desperate President Bashar Al Assad unleashes chemical weapons against his people as the rebels press their offensive inside Damascus? Or what if jihadists among the rebel ranks get hold of these poisonous weapons?
Both possibilities have been evoked with urgency in the last week by US media, which have spoken of "dozens" of shells and bombs being loaded with the deadly chemical sarin. The Assad regime's use against Syrian rebels of Scud missiles, which can deliver a chemical payload, have further raised concerns.
In a statement on Thursday, Russian Deputy Foreign Minister Mikhail Bogdanov warned that "extremists" might seize the chemical weapons stocks. US President Barack Obama has meanwhile served notice that if the regime made the "tragic mistake" of using chemical weapons, there would be "consequences" for which he would be held responsible.
That warning - reiterated by the UK - appears to have worked: it produced a new pledge from the Syrian government that it would never use chemical weapons against its own people.
The US defence secretary, Leon Panetta, said last week that there were no signs of "aggressive preparation" of the weapons by the Syrian military at present. But he did not say the threat had been eliminated.
According to a report on Friday by National Public Radio in the United States, dozens of shells and bombs have been loaded by the Syrian military with the nerve gas sarin "in the past several weeks", which the US radio network noted is a "larger estimate" than previously recorded. The Washington Post reported on Thursday that Mr Obama's warning had been prompted by surveillance photos showing at least one army unit loading specialised vehicles.
The paper, quoting two western officials, added that the government forces stopped the preparations "late last week" and there was "no evidence" that activated chemical weapons had been loaded onto aircraft or deployed to the battlefront.
One of the problems in reporting on the intelligence is that it is hard to tell how much information circulating in the media is fresh, and how much is a recycling of earlier intelligence that only serves to raise the stakes still higher. And of course, there is a question of whether the initial intelligence can be trusted in the first place, in the light of the politicisation and hype before the 2003 Iraq war.
But it would be a real matter for concern if there were evidence that the chemical components had been mixed, as they are stored separately for safety. Once combined, the chemicals have a short shelf life: in deterrence parlance, you have to "use them or lose them".
Syria never signed the 1993 Chemical Weapons Convention that outlaws the stockpiling and use of chemical weapons, whose horrendous effects were a hallmark of the Iran-Iraq war.
The Syrian government has never officially acknowledged that it possesses the toxic gases, although Syrian foreign ministry spokesman Jihad Makdissi may have hastened his own departure from Damascus by saying last August that Syria would never use chemical weapons against its own people, only against "external aggression". (Mr Makdissi disappeared from Damascus late last month, and is rumoured to have defected.)
Regardless, Syria is known to hold hundreds of tonnes of mustard gas, a blistering agent, and sarin gas, which attacks the central nervous system, causing convulsions and paralysis. It is also believed to hold stocks of VX nerve gas, which requires only a tiny droplet to touch the skin to kill in seconds.
According to the Federation of American Scientists, Syria has "one of the largest and most sophisticated chemical weapons programmes in the world" and may also have an offensive biological weapons programme.
Nato governments are not taking any chances. Nato has reached out to the western-backed Syrian National Coalition to warn of the risks. The military of the Czech Republic is sending experts with Soviet-era knowledge to train rebels in handling and safeguarding the weapons that are stored in a dozen sites across Syria, and the CIA is reportedly training rebels in Jordan to the same end. Nato has a contingency plan for dropping medicines to Syrian civilians if chemical weapons are used, according to NPR.
There are two further scenarios that should be considered: rebel groups may be tempted to turn to chemical weapons in case of infighting among the different groups following the sudden collapse of the regime and in the ensuing chaos. Or a renegade general may take it upon himself to unleash these fearsome weapons.
But there is no good strategic reason to turn to chemical weapons, which is possibly why the regime continues to insist that it will never do so. They are a weapon of mass destruction, often described as the "poor man's nuclear weapon". As such, they are indiscriminate and cannot be used in surgical strikes. They have not been used in conflict since the Iran-Iraq war, most notoriously by Saddam Hussein against his own people in Halabja in 1988.
But this is a new situation. Syria is engulfed in the first civil war in which significant quantities of WMD are known to be present. Western governments must be hoping and praying that their nightmare scenario doesn't become a reality.
Anne Penketh is an international security analyst based in Paris
On Twitter: @annepenketh
