WASHINGTON - As Bashar 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 El-Safir, which has served as a major production center for chemical 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 the main storage sites for mustard gas and nerve agents are supposed to be guarded by thousands of Syrian soldiers, but he predicted that they would be easily overrun.

"They're 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."

President Obama and other leaders have warned Assad not to use chemical weapons, calling that 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 ousted and Syria's chemical weapons are left unprotected or end up in the hands of anti-American insurgents.

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

U.S. defense officials 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, Turkey, Jordan and NATO allies to monitor dozens of sites where Syria is suspected of keeping chemical weapons and to coordinate options to intervene if necessary. 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 in 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.

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. "Securing the sites will be a big deal. We don't know how many sites there might be."