The federal safety agency - NHTSA - has opened an investigation into possible suspension failures on 2006-2008 Hyundai Sonata sedans.

An estimated 393,000 cars are involved, according to NHTSA, which told Reuters it received six complaints of the failure of rear suspension control arms due to corrosion.

Three of the reports "allege failures at highway speeds," NHTSA said. The agency said a preliminary evaluation will "assess the scope, frequency and potential safety consequences associated with the alleged defect."

Hyundai of America said in a statement: "It is too early in the process to draw any conclusions about the allegations, and no safety-related defect has been identified."

The automaker said it was "cooperating fully" with US officials.