US lawmakers reportedly have stepped up pressure on federal government regulators and Takata to speed up the recall of millions of airbag inflators, citing evidence the supplier manipulated data to cover up problems with its products.

Separately, according to a Reuters report, a group of 10 automakers said the cause of Takata airbag ruptures linked to 10 deaths and more than 100 injuries was a combination of exposure to humidity, design and manufacturing issues, and use of the volatile chemical ammonium nitrate.

The news agency reported earlier regulators may order a recall of a further 70m to 90m vehicles fitted with Takata airbag inflators.

Its latest report said a Senate Commerce, Science and Transportation committee report cited a series of internal Takata documents from the past 12 years that showed company officials argued data on inflator quality and testing was manipulated to disguise problems. A redacted 2013 document released by the committee indicated an unidentified Takata manager told the company's senior vice president of quality assurance that proposed limitations to the scope of a 2013 airbag recall might be "a violation of our moral obligation to protect the public".

US senator Bill Nelson cited the Monday Reuters report on the Senate floor, saying it was "puzzling" NHTSA had allowed Takata to continue production of ammonium nitrate-based inflators indefinitely.

The current recall may have to be redone, Nelson said, "because auto manufacturers are installing new live grenades into people's cars as replacements for the old live grenades."

Takata spokesoman Jared Levy told Reuters issues about testing and data handling raised by Nelson and a Democratic staff report "are entirely inexcusable and will not be tolerated or repeated".

"Issues with validation testing of the original phase stabilised ammonium nitrate inflators are not the root cause of the field ruptures...but these issues are totally incompatible with Takata's engineering standards and protocols," Levy said.

NHTSA in a statement on Tuesday cited a consent order issued last November under which "all Takata ammonium nitrate-based inflators must eventually be recalled unless Takata can establish their long-term safety".

The agency has repeatedly said that some replacement inflators "may be effective for several years, but not for the full life of your vehicle, and therefore may also have to be replaced". NHTSA has noted that all deaths and injuries reported took place in inflators at least seven years old, Reuters observed.

Former managers interviewed by the news agency described "chronic" quality failures at Takata's North American inflator plants, an assessment reflected in dozens of company emails and documents dating back to 2001. Those problems, the former managers said, make it difficult for the company and regulators to pinpoint which inflators, among tens of millions, pose a danger.

The review by the automaker-backed Independent Testing Coalition and a Utah-based team from Orbital ATK found the ruptures were caused by a combination of three factors: ammonium nitrate propellant without moisture-absorbing desiccant, long-term exposure to repeated high-temperature cycling in the presence of moisture and "an inflator assembly that does not adequately prevent the intrusion of moisture".

Takata said those findings were "consistent" with its own and those of Germany's Fraunhofer Group, which was hired earlier by Takata to conduct additional testing, Reuters said.

Newer inflators with drying agents, along with replacement airbags, will be considered, the report added.

The study only covered inflators that do not have a drying agent, or desiccant. Most of the inflators with ammonium nitrate that have not been recalled have the drying agent.

The consortium, which includes Honda, Toyota, Ford, General and Fiat Chrysler Automobiles, will next investigate the performance of all inflators being used as replacement parts for current recalls, Reuters said.

Rebecca Lindland, senior analyst for Kelley Blue Book, said: "The biggest challenge to this recall is getting the repairs done, and even then, those repaired vehicles may have to be reengineered if ammonium nitrate is found to be unsuitable.  This recall is not going away any time soon, and hopefully the death toll won't rise while the investigation continues. The toll on resources this recall is absorbing within the affected parties cannot be underestimated. NHTSA, automakers, Takata itself and other parties all will be tied up in this for years to come while the expense mounts."