WASHINGTON, March 18 (Xinhua) -- Faulty air bags in certain vehicles of South Korean automakers Hyundai and Kia have been under investigation, after car crashes in the United States killed four, U.S. road safety authority has said.

The agency "is currently aware of six crashes with significant collision related damage events involving Hyundai and Kia models where air bags failed to deploy in frontal crashes," said the U.S. National Highway Traffic Safety Administration (NHTSA) Saturday on the website of its Office of Defects Investigation.

The series of crashes resulted in four deaths and six injuries, it said, adding that the probe covers 2011 Hyundai Sonata and 2012-2013 Kia Forte cars, which could affect an estimated 425,000 cars.

Hyundai Motor Company was already aware of the faulty air bags as of Feb. 27, when it issued a recall for some 155,000 U.S. vehicles due to problems inflicted by an electrical over-stress in the air bag control unit. It has not yet identified a fix to the recall, the company said.

Hyundai has confirmed the fault in the airbag control circuitry in three crashes, while the fourth is still under investigation, the automaker said.

Its sister company Kia has not issued a recall so far, but said in a statement Saturday that it will cooperate with the NHTSA on the probe, although it has not confirmed problems of such in its 2002-2013 Forte cars.

The NHTSA said related air bag control units were supplied by German automotive company ZF-TRW, adding that the agency will determine if there are more car manufacturers using the same or similar units.