The U.S. auto safety agency has stepped up its probe into engine fires that have plagued some Hyundai Motor Company and Kia Corporation vehicles for over six years.

The National Highway Traffic Safety Administration (NHTSA) said it had opened an "engineering analysis" covering about 3 million vehicles to evaluate, among other things, the efficacy of recalls initiated by the two automakers. The agency added that it was aware of 161 fires occurring potentially due to engine failures.

An engineering analysis is the next step in a process that could lead to a recall, although sometimes NHTSA closes such probes without requiring any action.

The agency had opened an investigation in 2019 covering the 2011-2014 Kia Optima and Sorento and the 2010-2015 Kia Soul, along with the 2011-2014 Hyundai Sonata and Santa Fe to investigate instances of non-crash fires.

Hyundai and Kia said on Monday they continue to fully cooperate with NHTSA in regard to non-collision engine fires.

The two South Korean companies had agreed to a record $210 million civil penalty last year after regulators said they failed to recall 1.6 million vehicles for engine issues in a timely fashion.