The battle for Damascus seems to have begun. In one opposition video posted online today, I counted 19 rockets fired by Syrian government forces in one long terrifying salvo not far from the capital.

Another video shows a bomb falling from a Syrian air force jet. So what’s to stop the regime from going a step further and using chemical weapons, if nothing else halts the rebel advance?

Syria has the biggest chemical arsenal in the Middle East, and one of the biggest in the world, and opposition groups have long assumed it might be used.

They’ve produced lots of online guides on how to protect yourself from chemical attack, though one I looked at this afternoon showed how to make a gas mask with nothing more than cotton wool stuffed inside a paper cup.

It seems to me unlikely that President Assad himself would launch such an attack. Russian officials who have met him recently have been quoted as saying that he expects to be killed by his own people, with no hope of victory or escape.

But who knows what his security apparatus might do, or indeed what the most extreme of Syrian Islamist rebels might do, should weapons of mass destruction fall into their hands?

Now President Obama has himself spoken out, warning the regime not to use them. Words may be all the defence he has; one estimate is that it would take 75,000 troops to secure numerous Syrian chemical weapons sites – troops neither America’s President nor anybody else is prepared to commit. Bombing weapons silos from the air could launch deadly gases into the atmosphere.

04 damascus r w Chemical threat hanging over Syrias civil war

One is always wary of American intelligence reports on such arsenals in the Middle East, but US officials have been quoted as saying that weapons parts have been moved recently, and that chemicals have been mixed to make the nerve agent, Sarin – though Assad’s officials have repeatedly denied they would launch chemical attacks on their own citizens.

The most infamous denial came from Jihad Makdissi, the foreign ministry spokesman. In July he said WMD would not be used, but in doing so he came perilously close to admitting the weapons’ existence. This may have prompted his fall from grace in Damascus, which culminated in reports yesterday that he had fled and is now seeking asylum.

And if Mr Makdissi finds he can no longer promulgate his government’s line happily, why should anyone else believe it either?

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