DETROIT - Under pressure from government safety regulators, Ford Motor Co. is expanding a recall of the popular F-150 pickup truck to include nearly 1.2 million vehicles that may have defective air bags.

The additional recall, announced Thursday by the National Highway Traffic Safety Administration, covers trucks from the 2004 through 2006 model years. An electrical short can cause the air bags to deploy unexpectedly, in some cases injuring drivers.

In February, Ford agreed to fix 150,000 of the trucks but resisted the government's wishes to recall all 1.2 million trucks that may have the problem.

Ford's F-Series pickup truck is the top-selling vehicle in America, and the F-150 makes up about 60 percent of F-Series sales. Through March, the company sold nearly 127,000 of the pickups. The F-Series also includes heavier duty trucks such as the F-250 and F-350.

The Dearborn, Mich., company said in a statement that it will notify all owners in May that they should take their trucks to a dealer who will replace an air bag wire in the steering wheel. The repair takes less than a half day, the company said. The wire can become chafed, causing a short circuit that can lead to the airbag inflating unexpectedly.

Ford said it knows of no crashes caused by the airbag problem. But NHTSA said in a January letter to Ford that the agency knew of 269 cases in which the air bags deployed inadvertently, resulting in 98 injuries, some serious. The agency noted that Ford made production changes to the trucks in 2006 and 2007 to fix the air bag wiring and other issues.

Ford told NHTSA in May that some drivers reported injuries that included burns from contact with the air bag, bruises, neck and back pain and minor cuts. Two customers reported broken or chipped teeth and two reported elbow or arm fractures.

NHTSA wanted Ford to recall all 1.2 million trucks, but Ford told the agency that the full recall was not justified and said owners got an adequate warning of the problem from the air bag warning light on the dashboard. But NHTSA disagreed and said it could hold a rare public hearing on the matter. "The potential for loss of vehicular control poses an unreasonable risk to safety," NHTSA told the company.

Earlier this week, Ford officially agreed to the full recall. In an April 11 letter to NHTSA, James Vondale, director of Ford's safety office, said the company agreed to the expansion after discussions with NHTSA.

"To reassure customers of Ford's commitment to safety, and to eliminate any possible customer confusion, Ford is voluntarily recalling this remaining population of vehicles," Vondale wrote.

Ford shares fell 32 cents, or 2.1 percent, to $14.67 in morning trading.