Western spy agencies suspect Syria’s government has several hundred tons of chemical weapons and precursor components scattered among as many as 20 sites throughout the country, heightening anxieties about the ability to secure the arsenals in the event of a complete breakdown of authority in the war-torn nation, U.S. and Middle Eastern officials say.

Officials are monitoring the storage sites, but they expressed growing fear that they have not identified every location and that some of the deadly weapons could be stolen or used by Syrian troops against civilians.

“We think we know everything, but we felt the same way about Libya,” said a former American intelligence official who was briefed on U.S. preparations for both conflicts. “We had been on the ground in Libya, yet there were big surprises, both in terms of quantities and locations.” The former official was one of several people who spoke on the condition of anonymity to discuss classified information.

The collapse of government control in several Syrian provinces has prompted heightened scrutiny of the weapons depots by the United States and its allies in the region. It also has hastened preparations for securing the sites with foreign troops, the U.S. and Middle Eastern officials said.

Drawing from recent intelligence assessments, the officials believe that the Syrian arsenal contains several hundred tons of chemical weapons and precursors, including sizable quantities of battlefield-ready sarin, the deadly nerve agent.

The stockpile appears to be larger and more widely distributed than originally suspected, according to two officials who have seen the intelligence reports. They said the most dangerous chemical stocks are kept in bunkers in about a half-dozen locations, while as many as 14 other facilities are used to store or manufacture components.

Because of the risks posed by the stockpile, U.S. spy agencies have devoted enormous resources to monitoring the facilities through satellite imagery and to developing plans to safeguard the weapons if the crisis worsens, current and former U.S. officials said.

“It’s obvious that ensuring their security is paramount,” a U.S. official said. “Planning for different scenarios, consulting appropriately with allies and preparing to manage any new challenges is simply being responsible.”

Several current and former officials acknowledged the difficulty of securing chemical depots inside Syria, given the fighting underway and the likelihood of fierce resistance from Syrian forces to any incursions by outsiders. Some of the officials also conceded that there may be yet-undetected facilities within the country, roughly the size of Washington state.

The former U.S. intelligence official said North Korea and Russia have assisted Syria over the decades in constructing weapons facilities that are well-fortified and shielded from spy satellites. “They are masters at concealment,” the former official said.

In August, a Syrian Foreign Ministry spokesman, Jihad Makdissi, said the Damascus government would never use chemical arms against its people, but he warned that it would unleash the weapons against what he called foreign invaders. He said the military was guarding the stockpile.

In response, President Obama cautioned the Syrian government that any deployment of its chemical weapons would cross a “red line” and invite an immediate response by the West.

Syria is thought to possess the world’s third-largest stockpile of chemical weapons after United States and Russia, whose Cold War arsenals are being dismantled and destroyed. Syria’s weapons, predominantly deadly nerve agents that can be delivered by artillery rockets, shells and aircraft munitions, were developed for use in a war against Israel.

Heightened terrorism risk

The increased focus on Syria’s stockpile is driven in part by the government’s de facto retreat from large portions of the countryside as forces loyal to President Bashar al-Assad concentrate on driving rebels from Damascus, Aleppo and other key cities. Despite setbacks, the opposition Free Syrian Army says that up to half of the countryside is in rebel hands, a claim that underscores concerns that weapons depots could be abandoned or overrun.

U.S. and Israeli officials fear that the chemical sites could be looted, leading to weapons being sold or given to radical Islamists or to Iranian-backed Hezbollah fighters. A single crate of artillery shells or a few barrels of chemical precursors would contain enough lethal poisons for a series of terrorist attacks, weapons experts say.

Mitigating the risk somewhat is the fact that the most of Syria’s stockpile consists of chemical precursors that must be combined and loaded into shells or bombs. Amateurs who attempt to mix ingredients for sarin gas run a strong risk of killing themselves instead of their intended targets.

Still, the terrorism risk has prompted extensive contingency planning by the United States and regional allies, including Israel, Jordan and Turkey, said the U.S. and Middle Eastern officials.

Under the most optimistic scenario, teams of experts could be dispatched into rebel-controlled parts of Syria to secure and remove chemical weapons, as happened in Libya after the fall of Moammar Gaddafi. But if weapons sites are overrun during fighting — or if loyalist forces are seen preparing for a chemical attack — plans call for sending elite foreign military forces to secure the arms under fire, if necessary, according to the officials.

Fears of chemical attacks

Syrian rebels also have grown increasingly concerned about the stockpile, fearful not only about looting, but also about the possibility that Assad will use the weapons against rebel fighters and civilians as a last resort, said Andrew Tabler, an expert on Syria who recently returned from a month-long trip to the region.

“They recognize it as a problem,” said Tabler, author of “In the Lion’s Den,” a book about Syria under Assad. “They think the regime is moving the weapons around, mostly to the coast and other areas where the regime will go if it is forced to contract.”

The Obama administration shares the rebels’ concern that a desperate Assad might decide to use chemical weapons against his countrymen. But despite reports that Syria has consolidated some of its chemical stockpile in recent weeks, intelligence agencies have not detected evidence that Syrian forces are filling chemical shells or otherwise preparing for a chemical attack.