DETROIT/PARIS (Reuters) - U.S. auto sales boomed in August as $3 billion in government incentives drove sharp gains for Hyundai Motor Co and Ford Motor Co but failed to provide a boost for General Motors Co in its first full month outside bankruptcy.

Ford reported a 17 percent jump in monthly sales on Tuesday and the success of the U.S. government's "cash for clunkers" trade-in incentives pushed overall industry sales to the first year-on-year increase in 21 months. Korea's Hyundai posted a 47 percent increase as sales of its Elantra sedan more than doubled. The U.S. government incentive program also helped Honda post a 10 percent sales gain while Toyota Motor Corp sales were up 6 percent.

The two U.S. automakers to have emerged from government-sponsored bankruptcies -- GM and Chrysler -- lost market share during the August sales bonanza. GM sales dropped 20 percent, while Chrysler was off 15 percent. Nissan Motor Co sales fell almost 3 percent from record levels of a year earlier. Meanwhile, auto sales rose in France and Italy and stabilized in Spain on the success of similar government-backed sales incentives, data released on Tuesday showed.