As Bashar al-Assad’s hold on power steadily weakens, U.S. officials are increasingly worried that Syria’s weapons of mass destruction could fall into the hands of Islamist extremists, rogue generals or other uncontrollable factions.

Last week, fighters from a group that the Obama administration has branded a terrorist organization were among rebels who seized the Sheik Suleiman military base near Aleppo, where research on chemical weapons had been conducted. Rebels are also closing in on another base near Aleppo, known as Safirah, which has served as a major production center for such munitions, according to U.S. officials and analysts.

The opposition Free Syrian Army said it did not find any chemical weapons at the first installation. But the developments have fanned fears that even if Assad does not attack his own people with chemical weapons, he is on the verge of losing control of his formidable arsenal.

A former Syrian general who once led the army’s chemical weapons training program said that the main storage sites for mustard gas and nerve agents are supposed to be guarded by thousands of Syrian troops but that they would be easily overrun.

The sites are not secure, retired Maj. Gen. Adnan Silou, who defected to the opposition in June, said in an interview near Turkey’s border with Syria. “Probably anyone from the Free Syrian Army or any Islamic extremist group could take them over,” he said.

Timeline: Major events in Syria’s tumultuous uprising that began in March 2011.

President Obama and other leaders have warned Assad not to use chemical weapons, saying such a move would be a “red line” that would force them to take military action. But the White House has been vague about whether and how it would respond if Assad is toppled and Syria’s chemical weapons are left unprotected or end up in the hands of anti-
American insurgents.

Contingency planning

The Pentagon has drawn up plans for responding to possible scenarios involving Syria’s chemical arms, Defense Secretary Leon E. Panetta said Friday during a visit to Incirlik Air Base in Turkey, about 60 miles from the Syrian border. He declined to give details.

Defense officials, however, said in interviews that they have been updating their contingency plans in recent weeks as chaos has overtaken Syria. They said they are working closely with Israel, Jordan and NATO allies, including Turkey, to monitor dozens of sites where Syria is suspected of keeping chemical arms and to coordinate options to intervene if necessary.

Pentagon officials have described their plans to members of Congress in classified briefings. In public, military officials have indicated that they are preparing for potential joint operations with the Jordanian and Turkish armed forces, while sharing intelligence with Israel. U.S. officials also have sought to enlist the cooperation of Russia, which has a close military relationship with Syria and helped develop its chemical weapons program decades ago.

Meanwhile, the U.S. government and some European allies have hired private contractors to train Syrian rebels how to monitor and secure chemical weapons sites should Assad abandon or lose control of any of his stocks, according to CNN. A State Department spokeswoman declined to comment on the report.

“It’s safe to say it will take an international effort to secure the weapons,” Sen. Jeanne Shaheen (D-N.H.), a member of the Senate Armed Services Committee, said in an interview. Shaheen said she was “confident” about the administration’s contingency planning but warned that the task was formidable.

Shaheen noted that Panetta has described the challenge of securing Syria’s chemical cache as “100 times worse” than was required to safeguard Libya’s arsenal of chemical and conventional weapons after Moammar Gaddafi was toppled last year by a NATO-backed rebellion.

“What is challenging is that we have a situation that we don’t have control over,” she said. “The fighting is much more intense.”

In Libya, Gaddafi had a much smaller chemical stockpile, mostly precursor ingredients for mustard gas. Unlike Syria, Libya had signed an international treaty under which it had declared its chemical warfare materials and begun destroying them. Even so, in January, international inspectors discovered an undeclared cache of chemical munitions.

In a potential lesson for planning on Syria, the United States, NATO and other allies also were unable to secure Libya’s extensive stockpiles of rocket launchers and other conventional weapons, many of which were seized by militias or smuggled out of the country.

Although the Obama administration has been reluctant to become involved in the Syrian civil war beyond providing nonlethal aid to some rebel groups, nonproliferation analysts said no other country is likely to be able to supply enough trained personnel and specialized equipment to secure and dismantle the arsenal of chemical weapons.

“Who’s going to volunteer? Who’s going to cough up the funds? There aren’t a lot of countries that have this kind of expertise,” said Michael Eisenstadt, a retired Army officer who directs the military and security studies program at the Washington Institute for Near East Policy. “Everything has to be done very deliberately. Securing the sites will be a big deal. We don’t know how many sites there might be.”

Sensitive installations

Although the basic contours of Syria’s chemical weapons program have been described for years in congressional testimony and independent reports, analysts cautioned that the extent and precise nature of the stockpile remain a mystery.

Analysts estimated that the country has several hundred tons of mustard gas and highly toxic nerve agents, predominantly sarin. But there is also evidence that Syria has sought to develop a more lethal and persistent nerve gas known as VX, as well as the nerve agent soman.

Most of the chemical munitions are stored at two sites: a warehouse complex in Furqlus, outside the battle-stricken central city of Homs at the western edge of the Syrian desert, and an installation known as Khan Abu Shamat, about 50 miles east of Damascus, the capital. A third site is near Masyaf, west of Homs. Smaller stockpiles are thought to be scattered among dozens of other military installations nationwide.

The arsenal is so vast that it could take 1,000 outside inspectors and specialists just to monitor the condition of each site and take an inventory, said Leonard Spector, deputy director of the James Martin Center for Nonproliferation Studies at the Monterey Institute of International Studies.

That’s assuming there would be no need to provide security at the installations, much less engage hostile forces. In a worst-case scenario, under which the Syrian military would gas its own people, the Pentagon has projected that it could take up to 75,000 troops to intervene.

“It’s a manageable number in an environment where everybody’s cooperating,” Spector said. “Some of these other contingencies, the numbers add up fairly quickly.”

As the Syrian opposition steadily makes territorial gains, U.S. officials and analysts said the odds are increasing that insurgents will seize control of a chemical weapons site or that Syrian troops guarding the installations will simply abandon their posts.

“It’s almost inevitable,” Eisenstadt said. “It may have already happened, for what we know.”

Silou, the retired general who defected to the opposition, said Syria’s chemical weapons supply is capable of killing hundreds of thousands of people if deployed in heavily populated cities.

Gases and nerve agents could be packed onto 600 warheads, including mid-range, Russian-made Scud missiles, Silou said. Syria also has thousands of artillery shells capable of being fitted with chemicals, he added, or the poisons could simply be sprayed over populated areas or rebel outposts.

Silou said it was unlikely that insurgents would know how to successfully detonate chemical munitions on their own, but he worried that the weapons could be moved easily to Iran, to Hezbollah forces in Lebanon or to the al-Qaeda group in Iraq.

“After the regime falls, anyone could take them,” Silou said.

Only Assad has the authority to order the deployment of chemical weapons, Silou said. Ali Mamluk, director of Syria’s intelligence service, and Jamil Hassan, head of air force intelligence, also are authorized to issue the order — but only if Assad is dead.

The defector said he has proposed forming a 2,000-strong unit called the Mountain Heroes that could be deployed to guard the two largest chemical weapons depots if Assad is toppled.

The unit would be composed of officers who have defected and who specialize in the handling and use of chemical weapons. He said the unit would be pulled together from 200 defected officers now in Turkey, 1,200 in Jordan and the rest still inside Syria. But he said they lack money for rifles, uniforms, communication equipment and food.

Assad’s government has warned that rebels affiliated with al-Qaeda may be on the cusp of acquiring chemical weapons.

Last week, the Syrian Foreign Ministry said the al-Nusra Front — an anti-Assad group that has been labeled a terrorist organization by the United States and is also known as Jabhat al-Nusra — had seized a chlorine factory near the town of Safirah, east of Aleppo. “Terrorist groups may resort to using chemical weapons against the Syrian people,” the ministry cautioned.

The chlorine factory, known as the Syrian-Saudi Chemicals Co., is near the same town as a major chemical weapons production and research complex, also known as Safirah, analysts said. It is unclear whether munitions are stored there as well.

Morello reported from Antakya, Turkey.