US federal regulators have launched a formal safety defect investigation of the Chevrolet Volt after new battery fires in tests, the National Highway Traffic Safety Administration said Friday.

Earlier this month, US officials said they had launched an investigation into electric vehicle safety after a damaged lithium battery in a Volt caught fire three weeks after a crash test.

Nobody was hurt in the May fire, which damaged property at a government testing facility in Wisconsin.

The NHTSA then sought to recreate the May test, carrying out three tests last week on Volt lithium-ion battery packs, intentionally damaging the battery compartment and breaking its coolant line.

In two of the tests, the batteries caught fire, it said.

"NHTSA is therefore opening a safety defect investigation of Chevy Volts, which could experience a battery-related fire following a crash," the safety watchdog said in a statement.

"Chevy Volt owners whose vehicles have not been in a serious crash do not have reason for concern," the statement stressed.

There has so far been no recall, and the NHTSA said it had no reports of real-life crashes that led to battery-related fires in Chevy Volts or other vehicles powered by lithium-ion batteries.

Still "the agency is concerned that damage to the Volt's batteries as part of three tests that are explicitly designed to replicate real-world crash scenarios have resulted in fire," it stressed.

"If NHTSA identifies an unreasonable risk to safety, the agency will take immediate action to notify consumers and ensure that GM communicates with current vehicle owners," the statement added.

GM insisted in a statement that the vehicle is "safe and does not present undue risk as part of normal operation or immediately after a severe crash," and said it supported further testing by the NHTSA.

"GM and the agency's focus and research continues to be on battery performance, handling, storage and disposal after a crash or other significant event, like a fire, to better serve first and secondary responders," said GM's chief engineer for electric vehicles, Jim Federico.

"There have been no reports of comparable incidents in the field."