U.S. auto safety regulators are opening a formal investigation into fire risks in General Motors' Volt vehicles, the National Highway Traffic Safety Administration said on Friday.

The NHTSA said it was taking the action after recreation this month of a May crash test resulted in fires in two out of three tests.

While it is too soon to tell whether the investigation will lead to a recall of any vehicles or parts, 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, NHTSA said.

Earlier in November, the agency said it was looking into the safety of batteries used to power electric vehicles after fire broke out in a Volt battery pack three weeks after a side-impact crash test.

It said it was not aware of any fires resulting from actual crashes on roadways.

NHTSA said on Friday it believed electric vehicles have incredible potential to save people money at the pump, help protect the environment, create jobs and strengthen national security by reducing dependence on oil.

It is working with manufacturers to ensure they have appropriate post-crash protocols and to help inform emergency services of the potential for post-crash fires in electric vehicles.

The Volt is safe and does not present undue risk as part of normal operation or immediately after a severe crash, Jim Federico, General Motors chief engineer for electric vehicles, said in a statement.

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.

In the May crash test, the Volt's 400-pound lithium ion battery pack was damaged and a coolant line was ruptured.

Toyota Motor Corp's Prius, which dominates the hybrid vehicle market, is powered by older nickel metal hydride battery technology.

This month's tests aimed to simulate a real-world, side-impact collision into a tree or pole, followed by a rollover.

After a test on November 16 that did not result in a fire, a temporary increase in temperature was recorded in a test on November 17, NHTSA said on Friday. It said that battery pack caught fire.

During the test on November 18, using similar protocols, the battery pack was rotated within hours after being impacted and it began to smoke and emit sparks.

NHTSA said it was working with GM, and the Departments of Energy and Defense to further investigate.

The Volt has a gas-powered 1.4-liter engine to provide additional range after it has run about 40 miles on a fully-charged battery.

GM has sold about 5,000 Volts. The plug-in hybrid costs $40,000 before a $7,500 consumer tax credit.

(Reporting by Tim Dobbyn and Dave Clarke)

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