DETROIT — The U.S. government issued an urgent plea to more than 4.7 million people to get the air bags in their cars fixed amid concern that a defect in the devices can possibly kill or injure the driver or passengers.

The inflator mechanisms in the air bags can rupture, causing metal fragments to fly out when the bags are deployed in crashes. Safety advocates say at least four people have died from the problem and there have been multiple injuries.

Multiple automakers have recalled vehicles in the U.S. over the past two years to repair air bag inflators made by Takata, a Tokyo-based supplier of seat belts, air bags, steering wheels and other auto parts. In a statement Monday, the National Highway Traffic and Safety Administration warned owners of those cars to act right away.

The agency has been investigating the problem since June and has cited reports of six inflators rupturing, causing three injuries.

Worldwide, automakers have recalled about 12 million vehicles because of the problem.

The warning covers cars made by Toyota, Honda, Mazda, BMW, Nissan, General Motors and Ford. Passenger or driver air bags or both could have the faulty inflators. Safety advocates say the problem could affect more than 20 million vehicles in the U.S.

The rare action by federal regulators comes three weeks after a Sept. 29 crash near Orlando, Fla., that claimed the life of a 46-year-old woman. In that crash, Hien Thi Tran suffered severe neck wounds that could have been caused by metal fragments flying out of the air bag on her 2001 Honda Accord. Her Accord was among the models being recalled.

One police agency concluded the air bags caused her wounds, while another is still investigating. NHTSA is seeking information in the case.