US regulators have expanded a probe into a potential safety hazard in 1.4 million Toyota vehicles after nine people were injured in 161 crashes or fires.

No deaths were reported from the fires which began in the power window switch. Regulators did not provide details on the severity of the injuries in a notice posted on the National Highway Traffic Safety Administration website.

The investigation comes as Toyota is still working to regain US market share after its reputation was damaged by a series of mass recalls beginning in 2009 and then its supplies were hit by Japan's March 2011 quake and tsunami disaster.

Toyota nonetheless regained its position as the world's number-one automaker by sales in the first quarter of 2012, stealing back the lead from US giant General Motors.

This latest potential defect was discovered after certain Toyota vehicles experienced a "higher rate of fire and thermal event incidents," regulators said.

The affected vehicles include Toyota's top-selling Camry, the Camry Hybrid, the RAV4, Yaris and Highlander Hybrid, built between 2006 and 2008.

The investigation began in February and was upgraded to an "engineering analysis" on Thursday. It could, but may not necessarily, result in a recall.

Toyota also reported 49 warranty claims linked to fires in the power window switch.