Fears grow for fate of Syria's chemical weapons
The renewed upsurge of concern about Syria's chemical weapons stock-pile is a reminder of one of the most frightening aspects of the potential collapse of President Bashar al-Assad's regime.
Might the Syrian government seek to employ chemical weapons against rebel forces as a last resort ? It says that it would not use such weapons in this way.
But perhaps of even greater concern is what might happen to these weapons if the country descends into anarchy with nobody in effective control?
President Barack Obama's explicit warning to President Assad coincides with press reports citing an unnamed US official, claiming that the Syrian regime had begun mixing the chemicals required to make sarin nerve gas.
Within some 24 hours a second, different official was quoted by NBC News as insisting that there was in fact no evidence that the Syrian military had begun to mix the precursor chemicals needed to produce the deadly nerve agent.
So what is going on? Without access to the intelligence data it is almost impossible to say. Was there an element of political spin here to accompany Nato's decision to deploy patriot missiles in Turkey?
Sources contacted by the BBC say that there are indications of activity at certain chemical weapons storage sites.
However it is of course impossible to determine if this is a preliminary to the weapons' use or, as some analysts believe, much more likely, the movement of munitions to ensure their security. Indeed such movement has been noted in the past.
What is clear is that US and western intelligence agencies are monitoring Syria's chemical arsenal as closely as they can and for good reason.
There are even persistent reports in the US that preparations are being made to secure such stocks in the event of a regime meltdown.
One aspect of the problem is the scale and scope of Syria's chemical weapons programme.
Leonard Spector, executive director of the James Martin Center for Nonproliferation Studies based in Washington, notes that: "Syria has one of the world's largest chemical weapon arsenals, including traditional chemical agents, such as mustard, and more modern nerve agents, such as sarin, and possibly persistent nerve agents, such as VX.
"Syria is thought to have a number of major chemical weapon complexes, some in areas of current conflict, such as the Homs and Hama regions. The bases are said to be guarded by elite forces, but whether they would stay at their posts if the Assad regime collapses cannot be predicted."
An additional concern is the manner in which the different kinds of chemical weapons are stored.
Mr Spector notes that while the mustard agent is believed to be stored in bulk form, rather than in individual munitions, other agents are thought to be in "binary" munitions, in which two innocuous solutions combine when the munition is fired to create the chemical warfare agent.
These might be more easily transported and used than the bulk agent.
Mr Spector adds: "US officials believe Syria's chemical arms are stored in secure bunkers at a limited number of sites and have not been dispersed into the field."
Beyond the intelligence services there is little hard and fast detail on Syria's chemical weapons programme.
Unlike Libya, which had signed the Chemical Weapons Convention and was in the process of dismantling its stocks when Muammar Gaddafi's regime collapsed, Syria has not joined the convention and thus has never made any formal declarations of its stocks.
Indeed as Charles Blair, a Senior Fellow at the Federation of American Scientists underlines, Libya is not a terribly useful precedent when considering the potential problems surrounding Syria's chemical arsenal.
Libya's arsenal was much smaller; stocks of mustard agent were essentially old; locations of stockpiles were known and the Libyan authorities were co-operating in their destruction.
Crucially too, says Mr Blair, there are huge differences in the two countries' potential abilities to deliver chemical weapons.
"Libya was able to deliver its sole CW agent via aerial bombs only - a militarily ineffective manner in this case," he says.
"Syria, by comparison, is thought to possess a variety of platforms for chemical weapons delivery - an open-source CIA report lists aerial bombs, artillery shells and ballistic missiles."
There is considerable discussion as to the nature of the threat Syria's weapons pose.
Leonard Spector says that there are multiple dangers.
"Conceivably, the Assad government could use some of these agents against rebel forces or even civilians in an effort to intimidate them into submission," he says.
"Or insurgents could overrun one of the chemical weapon sites and threaten to use some of these weapons, in extremis, if threatened with overwhelming force by the Syrian army."
The scenario that is causing the greatest concern, he says, is the possible loss of control over Syria's chemical arsenal leading to the transfer of chemical weapons to Hezbollah, in Southern Lebanon, or to al-Qaeda.
Components of both organisations are now operating in Syria as one of the groups challenging the Assad regime, he says.
Such a link-up between al-Qaeda-affiliated groups and weapons of mass destruction has haunted US military planners for more than a decade.
In the face of such concerns there has been considerable pressure, not least from Washington, for the US to come up with plans to secure the Syrian weapons in the event of the collapse of the regime.
There has been a succession of press reports displaying various degrees of bravado suggesting US Special Forces are being readied to swoop in and take over Syria's chemical weapons infrastructure.
The reality is more complex. Such a mission would require significant numbers of "boots on the ground" in highly volatile circumstances.
As Charles Blair makes clear: "The Iraq experience demonstrates the difficulty of securing highly sensitive military storage facilities."
He argues that in Syria the challenges are likely to be greater "because no foreign army stands poised to enter the country to locate and secure chemical weapons manufacturing and storage facilities".
Of course, as Leonard Spector points out, details of US contingency planning are not known.
"The most desirable plan would be to urge the weapons' current custodians to remain in place during any transition of power, and to place the sites under the supervision of an international contingent that could monitor the weapons' security, as decisions were made about how to manage or destroy them in the future," he says.
However, he adds: "For the US to attempt to secure the sites in the face of armed resistance by Syrian forces would be extremely demanding, given the number of the sites involved and their considerable size."
Of course if the Assad regime were to go, a whole new set of issues emerges.
Would any new Syrian government agree to join the convention and agree to eliminate its chemical weapons stocks?
Or, as Leonard Spector notes, would they instead "insist on retaining them as a counter to Israel's nuclear capabilities and as a bargaining chip in future negotiations with Israel over the Golan Heights?"