U.S. auto sales remained flat in June, continuing the trend that started in May, due in part to a short supply of cars available from Japan. And even though General Motors posted a 10.5 percent increase in sales for June over the same month one year ago, the company's sales results were less than what analysts anticipated.

The average estimate by analysts pegged sales at GM (NYSE: GM) to rise as much 18 percent for the month.

GM was also expecting to perform better in the month across its brands, but Cadillac sales fell 7.9 percent as the brand cut back on deliveries to fleet car companies. Cadillac noted that sales to individual customers, a more profitable segment, increased 5 percent in the month.

GMC led GM's sales gain with a 14.9 percent increase, while its Buick brand increased 13.2 in June from the same month the previous year and Chevrolet posted a 10.9 percent gain. The company benefited from sales of more fuel-efficient cars. GM's stock opened lower on the news Friday briefly but quickly recovered, trading at $30.39.

Chevrolet's sales were advanced by strong response to the company's Cruze, a compact car. Other popular models were the Chevrolet Equinox and GMC Terrain compact crossovers.

Fleet sales at GM dropped one percent. For the first half of this year, GM sales are at 1.26 million, up 192,000 units from last year -- although last year remained a difficult period for GM as it was still in the process of emerging from bankruptcy.

German automaker Volkswagen sold 28,444 vehicles in the U.S. in June, up 35 percent from the same period a year ago. Sales of Volkswagen's Jetta increased 88 percent in the U.S.

Ford and Chrysler are expected to announce sales results for June later in the day. Analysts expect Ford to have solid increase, perhaps even showing the company gained marketshare in the U.S.

The average forecast of 41 economists surveyed by Reuters for all auto sales in the U.S. in June was 12 million vehicles, up from 11.1 million last year at the same time and 11.8 in May. U.S. auto sales have been sluggish in the U.S. this spring due to rising gas costs and a shortage of vehicles from earthquake and tsunami ravaged Japan as automakers Toyota, Honda and Nissan were unable to keep up in the damage with normal production activity.