Honda Motor Co. said on Thursday that it is expanding its U.S. response to potentially explosive air bags made by Takata Corp. , adding a small number of vehicles in certain hot and humid regions and upgrading its previous action to an official recall, reported The WSJ.

Honda’s move comes after rivals Toyota Motor Co. and Nissan Motor Co. took similar action last month over Takata air bags that could send out metal fragments into the cabin of cars. The issue is linked to as many as four deaths.

Honda said it is officially recalling vehicles equipped with the potentially defective Takata air bags in 13 high-humidity states and U.S. territories—specifically, Alabama, Florida, Georgia, Hawaii, Louisiana, Mississippi, South Carolina, Texas, Puerto Rico, U.S. Virgin Islands, Saipan, Guam, and American Samoa— and upgrading its previous action from a so-called safety-improvement campaign.

Honda also said it has determined the air bags contain a defect which if exposed to high-humidity conditions over long periods, can result in an “abnormal deployment.”

The specific number of vehicles under recall is undetermined at this point, the company said. The change to a U.S. recall status now requires Honda to abide by certain regulatory reporting requirements.

Honda said no injuries or fatalities have been confirmed for “the identified [vehicle] population related to this recall.”

The company’s move is in response to letters that the U.S. federal auto-safety regulator sent to 10 car makers and Takata in late October. In the letters, the National Highway Traffic Safety Administration requested that companies promptly recall vehicles in certain hot and humid regions, in some cases regions where they had already announced safety campaigns.

The NHTSA and Takata have said the air bags are at risk of exploding with too much force during a collision and spraying the driver and occupants with metal fragments.

In all, NHTSA has said 7.8 million vehicles made by 10 auto makers could be affected by the potentially defective Takata air bags and has urged customers to act immediately to notices sent by the manufacturers to make repairs. Of those vehicles, NHTSA has identified 5 million Honda and Acura cars.

The agency Takata have come under fire in recent weeks by lawmakers and safety advocates calling for a nationwide recall of all cars suspected to have the defective air bags.

Earlier this week, the NHTSA required Honda to produce documents pertaining to its eight recalls and service actions since 2008 involving Tataka air bags. Most of the cars involved are older-model vehicles. Honda says the repair involves replacing the air bag inflater on the passenger side.

The NHTSA since June has been investigating whether high humidity and temperature in certain regions had any impact on Takata air bags and inflaters.