By David Shepardson

WASHINGTON (Reuters) - General Motors Co <GM.N> said on Thursday it may be forced by U.S. regulators to recall another 4.3 million vehicles for potentially defective Takata air bag inflators, a call-back that would cost the U.S. automaker $550 million.

GM said in May it would recall 1.9 million vehicles for Takata Corp <7312.T> air bag inflators that may not work properly. In June it expanded that recall to another 600,000 vehicles after a review of ownership data.

GM said in a securities filing that the costs of replacing Takata air bag inflators in the 4.3 million additional vehicles would be $550 million, while replacing inflators in the 2.5 million vehicles recalled to date would cost as much as $320 million.

GM said it does not believe there is a safety defect in any of the 6.8 million vehicles, but agreed to the initial recall after talks with the U.S. National Highway Traffic Safety Administration.

In May, the NHTSA said 17 automakers will be required to recall another 35 million to 40 million U.S. air bag inflators assembled by Takata. Previously, 14 automakers had recalled 24 million vehicles with 28.8 million inflators linked to at least 13 deaths and more than 100 injuries.

GM said it believes that "the results of further testing and analysis will demonstrate that the vehicles do not present an unreasonable risk to safety and that no repair will ultimately be required."

GM has not begun making repairs on the initial 2.5 million vehicles recalled, and has not yet accounted for the costs of the recall.

GM said its Takata inflators have a unique design that does not pose a safety risk. The company said data showed no cases of an airbag rupturing among 44,000 deployments in large GM pickups and SUVs that contain Takata inflators.

Takata inflators used in GM trucks and large SUVs are designed with different venting for hot gases released when the airbag deploys, and they are installed in the vehicle in a way that minimizes exposure to moisture, GM said.

GM's stance is at odds with the position of regulators that all frontal Takata airbag inflators without a drying agent must be recalled.

NHTSA spokesman Bryan Thomas on Thursday reiterated the agency's position. "The science clearly shows that these inflators become unsafe over time, faster when exposed to humidity and variations of temperature," he said.

NHTSA on Thursday also posted guidance on how automakers can petition for permission to alter the recall schedule. NHTSA said the process should not be "used to eliminate a population of vehicles from the recall."

Upward of 100 million vehicles worldwide with Takata airbag inflators have been declared defective and are linked to 13 deaths and more than 100 injuries. Inflators can explode with too much force and spray metal shrapnel into vehicle passenger compartments.

(Reporting by David Shepardson; Editing by Jeffrey Benkoe and Diane Craft)