Jan 4 (Reuters) - FCA U.S. LLC (STLA.MI) reported a drop in U.S. auto sales on Tuesday, as the carmaker grappled with chip shortage issue that forced it to halt production across Europe and the United States. The fourth largest automaker in the world said annual U.S. sales fell by over 2% to 1,777,394 vehicles from 1,820,636 a year earlier.