DETROIT (Reuters) - The U.S. National Highway Traffic Safety Administration (NHTSA) on Wednesday ordered Japanese air bag supplier Takata Corp to expand its regional recall of driver-side air bags to cover the entire United States. NHTSA gave Takata until Tuesday to declare that its air bag inflators are defective and issue a recall. If it does not, NHTSA could begin steps to fine the Japanese firm up to $7,000 per vehicle, as well as forcing a recall. The regulator is limited and cannot fine a company more than $35 million. The U.S. Department of Transportation, which includes NHTSA, has asked Congress to raise that limit to $300 million. Shares in Takata plunged as much as 7.9 percent in volatile Tokyo trade and were down 2.7 percent by mid-morning on Thursday. A spokesman at Takata, which has resisted NHTSA's past requests that it expand the recall nationally, said the company was still reviewing the letter and could not immediately say what action it would take. Defective Takata air bags have been linked to at least five deaths, all in Honda Motor Co vehicles. A nationwide recall would affect vehicles made by at least five automakers: Honda; Ford Motor Co; Chrysler Group LLC, a unit of Fiat Chrysler Automobiles; Mazda Motor Corp and BMW AG. The Wednesday letter from NHTSA to Takata Senior Vice President Kazuo Higuchi said Takata's previous recall limited to areas of high humidity should be expanded because of incidents reported in California and North Carolina that are outside the initial regional recall area. Additional reporting by Chang-Ran Kim in TOKYO; Editing by G Crosse and Ian Geoghegan