(Adds Hyundai comment, details on crashes, supplier)
WASHINGTON, March 17 (Reuters) - The U.S. National Highway Traffic Safety Administration said on Saturday it is opening a probe into why some air bags failed to deploy in Hyundai and Kia vehicles after crashes in which four people were reportedly killed and another six injured.
The agency said it was reviewing 425,000 2012-2013 Kia Forte and 2011 Hyundai Sonata cars. It also said it will determine if any other manufacturers used similar air bag control units and if they posed a safety risk.
Hyundai Motor Co issued a recall last month for 154,753 U.S. Sonatas after non-deployment reports were linked to electrical overstress in the air bag control unit, but said it did not have a final fix yet.
Hyundai spokesman Jim Trainor said the South Korean automaker is aware of reports of two deaths in its vehicles, which he said occurred in head-on collisions at extremely high rates of speed. Trainor said the automaker is working with the NHTSA in the investigation, and that it has not seen any issues with any vehicle apart from the 2011 Sonata.
"We are working on getting the fix as fast we can," Trainor said.
NHTSA, which announced the start of the probe in documents posted on a government website, said it was aware of six crashes in which six people were injured when air bags failed to deploy in frontal crashes, including four in 2011 Hyundai Sonatas and two in 2012 and 2013 Kia Fortes. The 2013 Forte crash occurred in Canada.
The agency said the air bag control module was built by ZF Friedrichshafen-TRW, a German auto supplier that acquired TRW Automotive Holdings Corp in 2015. A ZF spokesman did not immediately respond to a request for comment on Saturday.
Kia Motors Corp did not immediately respond to a request for comment.
NHTSA said it believed that the 2012-2013 Forte cars also use similar air bag control units supplied by ZF-TRW.
The agency also said that electrical overstress appeared to be the root cause in a 2016 recall by Fiat Chrysler Automobiles of 1.4 million U.S. vehicles for air bag non-deployments in significant frontal crashes. (Reporting by David Shepardson Editing by Bill Trott and Paul Simao)