A US official, speaking on condition of anonymity to explain the sensitive discussions, said the United States still had no plans to put boots on the ground in Syria. President Barack Obama's administration has, in fact, so far refused to provide lethal support to the rebels fighting to oust President Bashar Assad's regime and the Pentagon has played down the possibility of implementing a no-fly zone anytime soon.
"There is not a imminent plan to deploy ground forces. This is, in fact, a worst-case scenario," the official said, adding US forces would likely play a role in such a mission.
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Two diplomatic sources, also speaking on condition of anonymity, said as many as 50,000 or 60,000 ground forces may be needed if officials' worst fears are realized, plus additional support forces.
Even a force of 60,000 troops, however, would not be large enough for peacekeeping and would only be the amount required to secure the weapons sites - despite some of the appearances of a Iraq-style occupation force, the diplomatic sources cautioned.
It is unclear at this stage how such a military mission would be organized and which nations might participate. But some European allies have indicated they are unlikely to join, the sources said.
The White House declined comment on specific contingency plans. Spokesman Tommy Vietor said that while the US government believes the chemical weapons are under the Syrian government's control, "Given the escalation of violence in Syria, and the regime's increasing attacks on the Syrian people, we remain very concerned about these weapons.
"In addition to monitoring their stockpiles, we are actively consulting with Syria's neighbors - and our friends in the international community - to underscore our common concern about the security of these weapons, and the Syrian government's obligation to secure them," Vietor said.
The Pentagon declined to comment.
Devastation in north Syria (Photo: AFP)
While there is no complete accounting of Syria's unconventional weapons, it is widely believed to have stockpiles of nerve agents such as VX, sarin and tabun.
The US official said there were potentially dozens of chemical and biological weapons sites scattered around the country.
Securing them could not be left to an aerial bombing, which could lead to the dispersion of those agents, the official said.
"There could be second-order effects that could be extremely problematic," the official said of aerial bombing.
Defense Secretary Leon Panetta said last month that it was important that Syrian security forces be held together when Assad is forced from power, citing, in particular, their ability to secure chemical weapons sites.
"They do a pretty good job of securing those sites," Panetta said in an interview with CNN in July. "If they suddenly walked away from that, it would be a disaster to have those chemical weapons fall into the wrong hands, hands of Hezbollah or other extremists in that area."
The United States, Israel and Western powers have been discussing the nightmarish possibility that some of Assad's chemical weapons could make their way to terror groups - al-Qaeda style Sunni Jihadi insurgents or pro-Iranian Shiite Lebanese fighters from Hezbollah.
Some Western intelligence sources suggested that Hezbollah and Iran's Revolutionary Guards, both close allies of Syria, might try to get hold of the chemical weapons in the case of a total collapse of government authority.
Syria began to acquire the ability to develop and produce chemical weapons agents in 1973, including mustard gas and sarin, and possibly also VX nerve agent.
Precise quantities and configurations of chemical weapons in the Syrian stockpile are not known. However, the CIA has estimated that Syria possesses several hundred liters of chemical weapons and produces hundreds of tons of agents annually.
The Global Security website, which collects published intelligence reports and other data, says there are several suspected chemical weapons facilities in Syria.
Analysts have also identified the town of Cerin, on the coast, as a possible production site for biological weapons.