Panasonic Corp., Tesla Motors Inc.'s primary supplier of lithium-ion battery cells for its electric Model S sedans, has not committed to investing in a massive U.S. battery plant proposed by Tesla's Elon Musk.

Joining Tesla's "Gigafactory" battery project would raise investment risks, Panasonic President Kazuhiro Tsuga told reporters at a briefing in Tokyo on Thursday.

Tesla, which announced plans for the facility in February, said it is reviewing potential sites in four states in the U.S. Southwest. The plant may require an investment of as much as $5 billion to build and employ about 6,500 people by 2020, the Palo Alto, California-based company, said.