WASHINGTON — Syria has started moving some parts of its huge stockpile of chemical weapons out of storage, American officials said Friday, but it was uncertain whether the transfer was a precaution as security conditions across the country rapidly deteriorated, or something more sinister.

Some analysts and lawmakers said Syria’s president, Bashar al-Assad, might use chemical weapons in a last-ditch attack against an increasingly potent rebel force, possibly as part of a campaign of ethnic cleansing. Other officials said Syrian security forces might be moving parts of the arsenal to prevent it from falling into rebel hands.

“The truth is, we just don’t know,” said one American official who has been monitoring intelligence reports since the Syrians began moving the chemical weapons in recent days. “There’s a big gaping hole in what we know.”

Over the past four decades, Syria has amassed one of the largest undeclared stockpiles of chemical and biological weapons in the world, including sarin nerve agent, mustard gas and cyanide, the officials said. It is unclear which agents are being moved, how big the transfers have been, and where the weapons are being taken.