U.S. intelligence agencies believe there is a strong possibility the Assad regime will use chemical weapons on a large scale as part of a last-ditch effort to protect key Syrian government strongholds if Islamist fighters and other rebels try to overrun them, U.S. officials said.

Analysts and policy makers have been poring over all available intelligence hoping to determine what types of chemical weapons the regime might be able to deploy and what event or events might trigger their use, according to officials briefed on...