A Toyota spokesman said the National Highway Traffic Safety Administration investigation "involves potentially" 830,000 vehicles, with the focus on Camrys and RAV4s of the model year 2007.
The carmaker said it had received a preliminary notice of the issue from the NHTSA this week.
"However, we have not received an Information Request from the NHTSA. Toyota will fully cooperate with the agency in its efforts to investigate these allegations," he said.
The concerns came from six cases submitted to the NHTSA of fires erupting in the driver-side doors of Toyota vehicles.
Toyota's reputation was hit hard in 2009-2010 when it had to issue a recall for some 10 million vehicles around the world, most of them in the United States, for a series of technical problems.