WASHINGTON — Plagued by poor morale as well as fuel and food shortages, some Russian troops in Ukraine have surrendered en masse or sabotaged their own vehicles to avoid fighting, a senior Pentagon official said on Tuesday.

Some entire Russian units have laid down their arms without a fight after confronting a surprisingly stiff Ukrainian defense, the official said. A significant number of the Russian troops are young conscripts who are poorly trained and ill-prepared for the all-out assault. And in some cases, Russian troops have deliberately punched holes in their vehicles’ gas tanks, presumably to avoid combat, the official said.

The Pentagon official declined to say how the military made these assessments — presumably a mosaic of intelligence including statements from captured Russian soldiers and communications intercepts — or how widespread these setbacks may be across the sprawling battlefield. The official spoke on the condition of anonymity to discuss operational developments.