General Motors Co will allow Chevrolet Volt owners to drive other GM vehicles while U.S. regulators investigate the safety of batteries used in the Volt after crash tests produced fires.

Senior GM engineers will work with the National Highway Traffic Safety Administration to explore changes to the battery, the U.S. automaker said on Monday.

Last week, the National Highway Traffic Safety Administration opened a formal defect investigation of the Volt to assess the fire risk.

The agency conducted a side-impact crash test for the Volt in May that produced a fire more than three weeks after the test.

Efforts this month to reproduce that problem also resulted in a delayed fire, prompting NHTSA's formal probe.

GM is also exploring further design changes to strengthen the current battery pack, Mary Barra, head of GM global product development said during a conference call with reporters.

Volt drivers can arrange for a free GM vehicle loan until the issue is resolved with NHTSA.

A vehicle loan program of this nature is well beyond the norm for a preliminary investigation, and it underlines our commitment to the vehicle and its owners, said Mark Reuss, GM North American president, in a statement.

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

(Reporting by Kevin Krolicki and Deepa Seetharaman; Editing by Tim Dobbyn)