WASHINGTON (Reuters) - The National Highway Traffic Safety Administration said Wednesday it was standing by its initial decision that 51 million air bag inflators in 49 million U.S. vehicles built by 13 automakers pose serious safety risks.

NHTSA argued at a hearing in October that inflators produced by the two air bag manufacturers, ARC Automotive and Delphi Automotive, should be recalled because they may rupture and send metal fragments flying. After automakers raised objections in December, the agency did not immediately finalize its decision. The agency said Wednesday it was supplementing its initial determination and giving automakers another 30 days to respond before it could finalize the determination and formally demand recalls.