DETROIT — The U.S. government is telling 3 million more car owners to get their air bags repaired immediately, but its message has generated some confusion about which cars are actually affected.

The government’s auto safety agency is now warning 7.8 million car owners that inflator mechanisms in the air bags can rupture, causing metal fragments to fly out when the bags are deployed. The original warning Monday covered 4.7 million vehicles.

Safety advocates say at least four people have died from the problem, which they claim could affect more than 20 million cars nationwide. The inflators are made by Japanese parts supplier Takata Corp.

Car owners might experience some uncertainty, however, in determining if their vehicle is equipped with the potentially dangerous air bags. The warnings from the National Highway Traffic Safety Administration cover certain models made by BMW, Chrysler, Ford, General Motors, Mazda, Honda, Mitsubishi, Nissan, Subaru and Toyota.

Most of the 7.8 million vehicles are subject to existing recalls. But manufacturers have limited the recalls to high-humidity areas, excluding cars and trucks in states to the North. NHTSA says owners in Florida, Puerto Rico, Guam, Saipan, American Samoa, the U.S. Virgin Islands, Hawaii and “limited areas near the Gulf of Mexico in Texas, Alabama, Mississippi, Georgia, and Louisiana” should pay special attention to the warning.

Worse yet, the regulatory agency has twice corrected the number of vehicles affected and admitted that a list it released Monday included some cars not equipped with Takata air bags while omitting others that have them. The agency urged people to use its website see if their cars are affected — but a feature allowing people to check for recalls by vehicle identification number malfunctioned Tuesday and still wasn’t operational Wednesday morning.