Transportation Department's safety agency issued a bulletin to owners of 4.7 million vehicles made by Toyota and five other automakers, highlighting an apparently higher level of danger from the faulty air bags for car owners in areas with high humidity.

"The National Highway Traffic Safety Administration urges owners of certain Toyota, Honda, Mazda, BMW, Nissan, and General Motors vehicles to act immediately on recall notices to replace defective Takata air bags," the NHTSA said.

"The message comes with urgency, especially for owners of vehicles affected by the regional recalls in the following areas:  Florida, Puerto Rico, Guam, Saipan, American Samoa, Virgin Islands and Hawaii."

The air bag could improperly inflate and rupture, potentially sending shrapnel into the car's occupants.

Honda has the highest number of vehicles covered by the recall at roughly 2.8 million. Toyota is second at 778,000, followed by BMW at 574,000.

The high-profile alert came as Toyota Motor Sales USA said it was recalling Toyota Corolla, Matrix, Sequoia, Tundra and Lexus cars, sport utility vehicles and pick-up trucks produced from 2001 to 2004 to replace the air bag inflator for the front passenger seat.

Toyota expanded on an earlier airbag-related recall especially to include vehicles in high humidity areas, including southern Florida, Gulf Coast states, Hawaii and Puerto Rico, where the vehicles "appear to warrant immediate action" based on testing by Takata.

According to Toyota, Takata is still testing whether high humidity is an important factor in the airbag problem.

Toyota said it had received no reports yet of injuries or fatalities related to the airbag problem.

Still, the seriousness of the risk to occupants of the front seat was underscored in the company's details on the recall process .

If a replacement inflator part is not available when the vehicle is brought in for servicing, the dealer will temporarily disable the front passenger air bag.

In that case, the dealer will install a warning that the front passenger seat should not be occupied until the repair has been completed.

Toyota's US-traded shares were up 3.1 percent at USD 113.24 in afternoon trade on the New York Stock Exchange.