BEIRUT (AP) -- The U.S. and regional allies are closely monitoring Syria's chemical weapons -- caught in the midst of a raging civil war -- but options for securing the toxic agents stuffed into shells, bombs and missiles are fraught with risk.

President Bashar Assad's embattled regime is believed to have one of the largest chemical weapons stockpiles in the world. Fears have risen that a cornered Assad might use them or that they could fall into the hands of extremists, whether the Lebanese Hezbollah militia, an Assad ally, or al-Qaida-inspired militants among the rebels.

For now, the main storage and production sites are considered secure. However, some suggest the civil war poses one of the gravest risks of losing control over non-conventional weapons since the breakup of the Soviet Union two decades ago.

Syria's suspected arsenal is scattered across a number of locations, mainly in the north and west, where fighting between Assad's forces and rebels seeking to oust him has been heaviest.

"We need to be up front that this is not something very easy to do," Steven Bucci, a former senior Defense Department official, said of attempts to keep the weapons locked up.

The price of military action against the arsenal is prohibitively high, Bucci and others say.

Airstrikes on chemical weapons depots could inadvertently release toxic clouds or expose them to looters. A ground operation would require thousands of troops, and the U.S. administration has pushed back on any suggestion of direct military action in Syria. Pinpoint operations by special forces could easily go wrong.

The issue has been a topic in the U.S. presidential campaign. Republican nominee Mitt Romney has said he would send U.S. troops into Syria if needed to prevent the spread of chemical weapons, while President Barack Obama has said that movement or use of chemical weapons would have "enormous consequences."

Syria's secrecy compounds the problem. Damascus hasn't signed non-proliferation agreements, long denying it has chemical weapons. Syria "is a black hole for us," said Michael Luhan of the Organization for the Prohibition of Chemical Weapons, declining to give an estimate of the size of the arsenal because foreign inspectors are barred.

Other experts acknowledge there is no firm data and say they base their estimates largely on U.S. intelligence reports.

Syria is believed to have hundreds, if not thousands, of tons of chemical agents, said Leonard Spector, deputy director of the James Martin Center for Nonproliferation Studies in Monterey, California. This includes mustard gas, a blistering agent, and the more lethal nerve agents sarin and VX, he said.

The chemical agents are believed to be designed for use in artillery shells, aerial bombs and ballistic missiles, said Scott Stewart of the U.S. security think tank Stratfor.

It is not known to what extent the chemical agents have already placed in munitions. Bucci, of The Heritage Foundation, said he believed "most of it" has been put into artillery shells and rockets.

Bucci and Stewart estimate some 50 sites are associated with the program.

A map by the Monterey think tank shows four production sites: one 20 kilometers (12 miles) southeast of Aleppo, Syria's largest city and a major battleground, and three outside the cities of Hama, Homs and Latakia. Storage sites have been identified near Hama, Homs and the capital Damascus, which also has a research and development facility. Three sites are marked as having dual use infrastructure, for both civilian and military purposes.

Anxiety rose over the summer after the regime warned it might use chemical weapons against foreign attackers. Obama warned Assad that the threat of chemical warfare is a "red line" for the U.S. Even key Assad ally Russia told him to stand down.

Syria has not used chemical weapons, unlike Iraq's former leader Saddam Hussein. Analysts say the bigger threat is that the weapons fall into the wrong hands.

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."