A secretive Syrian military unit at the center of the Assad regime's chemical weapons program has been moving stocks of poison gases and munitions to as many as 50 sites to make them harder for the U.S. to track, according to American and Middle Eastern officials.

The movements of chemical weapons by Syria's elite Unit 450 could complicate any U.S. bombing campaign in Syria over its alleged chemical attacks, officials said. It also raises questions about implementation of a Russian proposal that calls for the regime to surrender...