US safety investigators said Thursday they are still uncertain about the precise cause of deadly ruptures in Takata auto airbags, but said they are pushing to accelerate a replacement campaign.

Officials of the National Highway Traffic Safety Administration, which oversees auto safety, said the premature explosions of airbag inflators is linked to volatile ingredients, age, and use in hot and humid climates.

But what precisely happens to cause the explosions, which can send inflator and vehicle shrapnel into occupants, is still unclear, said Scott Yon of the NHTSA Office of Defects Investigation.

Despite efforts "by some of the best and brightest minds in science and engineering, the root cause of the rupture remains unknown at this time," Yon said in a public presentation on the problem and recall operations.

In the United States there have been seven deaths and scores of injuries from 121 unintended eruptions of the Japanese-made airbags, most of them driver-side airbags.

The problem has also been reported in other countries, with at least one other death tied to it.

NHTSA chief Mark Rosekind said the agency could expand the recall if it finds it necessary, and that it was working on ways to speed up the replacement of the inflators on cars already slated for recall. Yon said the entire job would still take more than six months.

The issue is a challenge due to the difficulty of both understanding the cause and ramping up production of replacement inflators for tens of millions of cars worldwide, Rosekind said. Cars of at least 12 automakers are affected.

"The Takata issue is unprecedented in its size and complexity," Rosekind said.

Some 23 million airbags need inflator replacements in US cars alone, with four million cars requiring both driver-side and passenger-side airbags to be addressed.