General Motors staunchly defended the Chevrolet Volt on Monday, even as it offered to loan replacement vehicles to any Volt owner concerned about a federal inquiry into the safety of the car’s battery pack.

GM responded assertively and candidly to what the National Highway Traffic Safety Administration called “a formal safety defect investigation” into the Chevrolet Volt’s lithium-ion battery. The probe, announced Friday, was prompted by two fires that followed crash tests. The agency said it is not aware of any real-world crashes causing fires but is “concerned” that tests “explicitly designe...