DETROIT (AP) – General Motors is offering free loaner cars to Chevrolet Volt owners worried about the vehicles catching fire.

The move comes after the National Highway Traffic Safety Administration announced Friday that the government is investigating fires involving the Volt's lithium-ion batteries.

NHTSA said a Volt battery pack that was being monitored after a crash test caught fire on Thursday. The agency says another battery that recently was crash-tested gave off smoke and sparks. The latest fires are in addition to a battery fire at a test facility in Wisconsin back in June.

GM said on Monday that the electric cars are safe but it will contact owners of the more than 5,000 Volts sold in North America since December of 2010 to reassure them. It will also offer the loaner cars to ensure that Volt owners are satisfied and confident in their purchase. GM has not put a time limit on how long customers can keep the loaners.

The Volt, which can travel about 35 miles (56 kilometers) on electric power before a small gasoline generator kicks in to run the car, has helped Chevrolet's public image, and GM is eager to protect that good will. The company has promoted the car extensively as a first step toward independence from foreign oil. The car has also helped counter GM'S gas guzzling image left over from years of selling mainly pickup trucks and inefficient sport utility vehicles.

The company put two top executives on a conference call Monday to announce the loaner car program and answer questions about the fires.

Mary Barra, GM's senior vice president of product development, said both fires reported by NHTSA occurred seven days to three weeks after the crash tests, and could have been prevented if the battery charge had been drained as GM has called for in its post-crash procedures.

She said only a few Volts have crashed on public roads, and none of those has caught fire, nor have the battery packs been compromised.

"We don't think there's an immediate fire risk," said GM North American President Mark Reuss, who was also on the call. "This is a post-crash activity."

GM officials have said previously that NHTSA wasn't aware of the post-crash procedures at the time of the June fire. Barra said that in all the incidents, the battery cell was not involved in the fires, only the electronics within the battery. But she would not be more specific until NHTSA's investigation is over, she said.

In the Volt's system, Lithium-ion battery cells are assembled into a battery pack, and coolant is pumped between the cells to keep them from overheating. In the June fire at a test facility in Burlington, Wisconsin, coolant leaked from the battery and crystallized, and that could have been a factor in the fire, GM has said. The fire came three weeks after a side-impact crash test and was severe enough to cause several other vehicles parked nearby to catch fire as well.

Reuss said GM won't sell any Volts in other countries until it makes sure emergency responders, salvage yards and dealers have been trained to discharge the batteries after a severe crash.

In the U.S., GM is notified of any severe Volt crashes through its OnStar safety system, and it sends a team to the car within a day to drain the battery charge to prevent any fires.

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