U.S. safety regulators do not expect Japanese auto parts maker Takata Corp to be able to fully supply replacement parts for millions of defective air bags until January or later, and urged Takata and the automakers it supplies to seek additional parts from other companies, reported Reuters.

The National Highway Traffic Safety Administration, which has come under fire for being too lax and slow in dealing with the defective air bags, also called on automakers to be ready to expand their recalls beyond the warm and humid regions believed to be the epicenter of the problem.

The defective air bags, which can launch metal shards into car occupants, have been tied to at least four deaths and many serious injuries. They have triggered the recall of more than 10 million vehicles since 2008 by 10 different manufacturers.

Takata must boost production and even tap competitors to address demand while car makers must do much more to aid the recall effort, a top safety official said in letters to manufacturers.

"More can and should be done as soon as possible to prevent any further tragedies," the agency's deputy director, David Friedman wrote to auto makers.

Car makers should boost advertising to alert drivers of air bag dangers and offer loaner cars during repair, he said. He said car makers should even test Takata air bags themselves, as regulators keep studying why the safety device has been involved in so many mishaps.

If test results indicate potential defects with inflators in other parts of the country, "you will need to act quickly to expand your recall," he wrote, also asking if the companies are seeking to buy replacement parts from other suppliers.

Friedman will meet with Takata executives on Thursday and expects weekly updates on progress, the official wrote.

Regulators reached out to General Motors, Nissan, Ford, BMW, Honda, Mazda, Chrysler, Mitsubishi, Subaru and Toyota.

Separately on Thursday, lawmakers called for an independent review of NHTSA and asked whether the agency could do more to detect and snuff out design dangers. The agency briefed U.S. lawmakers earlier this week.

The agency released summaries of three meetings with Takata and vehicle manufacturers over the past three months. The summaries indicate that an estimated 3.1 million inflators would need to be replaced as part of the regional recall announced in June. By September, that estimate had grown to 4.3 million inflators.

In a letter to Takata Senior Vice President Kazuo Higuchi, NHTSA Deputy Director David Friedman demanded that Takata provide updated estimates of its ability to produce replacement parts and called on the Japanese firm to provide details about a program to test inflators "as soon as possible."