Such worries over the fate of advanced weaponry were highlighted on Friday, when a shadowy militant group known as Jabhat al-Nusra joined Syrian rebels in seizing a government missile defense base.
U.S. Defense Secretary Leon Panetta said late last month that Washington believes the main sites are secure but the regime apparently moved some chemical weapons to protect them. Panetta acknowledged that the U.S. doesn't know what happened to some of the weapons.
Spector told Congress this summer that the regime could lose control over chemical weapons sites, even as it holds on to Syria's urban centers. The rebels control stretches of countryside in the north and the west, close to where the main production facilities are believed to be, said Spector, a former senior U.S. arms control official. With front lines shifting, such sites could fall behind rebel lines or its regime guards could abandon them.
Hezbollah fighters, meanwhile, could take advantage of the chaos and try to loot installations. Israel, which fought a war with Hezbollah in 2006, has warned it would act, presumably by striking suspicious Hezbollah convoys.
However, the possibilities for military action are limited because of the size and decentralization of Syria's arsenal. Bucci and Stewart said airstrikes carry too much risk of harming civilians, while targeted operations would not be able to secure all sites simultaneously.
Using special forces "would necessitate putting troops in harm's way, without overwhelming support," said Stewart, a former anti-terrorism investigator at the U.S. State Department. "The only way to secure all the sites in a comprehensive manner is through a large ground force, which is politically untenable at this point."
Technical and political restraints could decrease the risks of militants obtaining and using chemical weapons.
Militant groups may lack the proper gear, training and logistics to move chemical weapons, said Michael Eisenstadt of the Washington Institute for Near East Policy. Some chemicals are stored in heavy bulk containers, while so-called binary munitions for missile warheads require separate components that are likely stored separately, he noted.
Smaller munitions, such as an artillery shell filled with chemical agents, would be easy to move, Bucci said. Militants could "fit it in a suitcase, carry it around and use it by hooking it up to other munitions," he said.
Hezbollah could be deterred by the threat of Israeli retaliation, said Stewart. Such payback would jeopardize Hezbollah's standing as a key military and political force in Lebanon.
"The largest concern is jihadist actors getting their hands on chemical weapons munitions and using them in the region," such as firing rockets at Israel or targeting Western diplomatic missions in the area, he said.
For now, the West's best options are deterrence and containment, analysts said.
This includes warning the regime and the rebels of the dire consequences of using or losing control of chemical weapons and working with Syria's neighbors, particularly Jordan and Turkey, to prevent chemical weapons from being smuggled out of Syria.
On Thursday, Jordanian officials confirmed that U.S. special operations forces and their Jordanian counterparts have been training at a compound some 80 kilometers (50 miles) from the Syrian border how to protect civilians from possible chemical attacks.
"With chemical weapons, it starts to get so beyond the pale," Bucci said of the potential threat. "It scares the heck out of everybody, rightfully."