WASHINGTON U.S. auto safety regulators said Monday they had opened a probe into 16 separate recalls issued by Hyundai and Kia covering 6.4 million vehicles relating to brake fluid leaks that could result in fires.

The Korean automakers have issued a string of recalls since 2016 for antilock braking system issues and fires.

The National Highway Traffic Safety Administration said it was opening an audit query to evaluate the timeliness of Hyundai and Kia’s defect decision making "and adherence with reporting requirements; and understand the varying defect descriptions and remedies between these recalls."

In September, the Korean automakers recalled a combined 3.37 million vehicles in the United States due to the risk of engine fires, telling owners to park outside and away from structures until repairs are complete.

(Reporting by David Shepardson; Editing by Bernadette Baum)