US regulators said on Monday they could expand their investigation into Takata Corp air bag inflators beyond 11 automakers, as questions arose about whether vehicle design played a role in the devices posing a deadly risk to the public.

National Highway Traffic Safety Administration (NHTSA) chief Mark Rosekind said his agency can add more auto manufacturers to the consent order that regulators announced with Takata in May. He said some car makers could also begin to move more quickly when they suspect a potential problem.

On Thursday, NHTSA expects to make a case in public that it should coordinate the Takata recall to ensure that an estimated 23.4 million air bag inflators installed in 19.2 million US vehicles from 11 automakers are properly replaced.

"We'll try to be very specific on Thursday but it goes beyond the 11," Rosekind told reporters.

"All of these are fitting under the investigation we currently have. And we'll be talking about all of those," he said.

The air bag inflators, which can explode with too much force and spray metal shrapnel into passenger compartments, have been linked to at least eight deaths and more than 100 injuries worldwide.

Rosekind's comments follow news in August that regulators directed the US units of Volkswagen AG to provide information on the June rupture of a Takata side air bag in a 2015 model VW Tiguan. The report does not fit the pattern of other cases, which have involved front air bags of older model vehicles.

Takata declined on Tuesday to comment on which automakers' cars could be added to the list under investigation. The list already includes vehicles manufactured by Toyota Motor Corp, Honda Motor Co Ltd and General Motors.

"We're concentrating on producing replacement parts and we're working with automakers to contain the issue. We plan to continue our co-operation with NHTSA," a company spokeswoman said.

Shares in the embattled Japanese air bag maker fell as much as 4.2 per cent to a two-week low of 1,352 yen ($11.31) on Tuesday morning on concerns that an ongoing global recall of potentially faulty airbags could expand to more companies.

Rosekind also said he believes automakers could follow the lead of GM by moving quickly on Takata-related recalls as additional attention is focused on the danger.

"It would not surprise me if other people decide to deal earlier and faster with these issues," Rosekind said.

Over the weekend, GM recalled about 400 vehicles in the US after being notified by Takata that tests at a production plant in Mexico showed failures of side air bags. The Takata tests were performed on October 5, and it notified GM the following day.

The GM recall affects six models produced earlier this year, mainly in February and March, including the Chevrolet Equinox and Chevrolet Malibu. No injuries or deaths have been reported in this specific recall, a GM spokesman said.

Experts say the ammonium nitrate used as a chemical accelerator in the inflators could become unstable after being exposed to high humidity over a period of time. Some have suggested that auto design, including how well sealed passenger compartments are against humidity, could also be a factor.

“Think about age, high humidity, all the different factors. Now you add design of the vehicle,” Rosekind said.

Newer Takata inflators have not been shown to be a problem, he added.

“So far the testing is showing that the newer ones seem to be solid,” Rosekiind said.

Takata said in a news release on Monday that it has increased production of air bag replacement kits to 1 million per month. - Reuters