(Add share prices, S. Korea investigation)

By David Shepardson

WASHINGTON, April 1 (Reuters) - The top U.S. auto safety regulator said on Monday it will open an investigation into 3 million Hyundai Motor Co and Kia Motors Corp vehicles after reviewing reports of more than 3,000 fires that injured over 100 people.

The National Highway Traffic Safety Administration (NHTSA) said the investigation is in response to a petition seeking a probe filed in June by the Center for Auto Safety.

The investigation covers the 2011-2014 Kia Optima and Sorento and the 2010-2015 Kia Soul, along with the 2011-2014 Hyundai Sonata and Santa Fe.

The probe rekindles safety concerns about vehicles of the South Korean duo, who have already been investigated by the U.S. regulator and prosecutors over engine-related recalls.

Hyundai and Kia together have recalled over 2.3 million vehicles since 2015 to address various engine fire risks.

Hyundai Motor shares fell as much as 2.5 percent in morning trade in Seoul, while Kia Motors fell 1.1 percent.

Hyundai said in a statement it was cooperating and added it has been in "frequent, open and transparent dialogue regarding non-collision engine fires."

Kia said it will continue to work with NHTSA and "openly shares information and data with NHTSA on all matters pertaining to vehicle safety."

NHTSA said the decision to initiate the probe was based on its analysis of information received from multiple manufacturers, consumer complaints and other sources. The agency said while prior Kia and Hyundai vehicle recalls covered by the probe are primarily related to engine fires, the new probe "is not limited to engine components and may cover additional vehicle systems or components."

Jason Levine, executive director of the Center for Auto Safety, said it was "long past time for the full power of the federal government to be brought to bear to answer why so many thousands of Kia and Hyundai vehicles have been involved in non-crash fires."

He added he hoped the probe will quickly lead to new recalls. "The evidence is now clear - Hyundai and Kia should have acted to recall these vehicles far earlier," he said.

Last month, Connecticut Attorney General William Tong said a group of U.S. states is investigating Hyundai and Kia for potential unfair and deceptive acts related to reports of hundreds of vehicle fires.

In November, Reuters reported federal prosecutors had launched a criminal investigation into Hyundai and Kia to determine if vehicle recalls linked to engine defects had been conducted properly.

South Korean prosecutors are also conducting separate investigations into the automakers over the recalls, raiding their offices and summoning executives for questioning.