WASHINGTON (Reuters) - U.S. wholesale inventories rose less than initially thought in August amid a sharp moderation in the pace of increase in motor vehicle stocks, a trend that if sustained could temper expectations for robust economic growth in the third quarter.

The Commerce Department's Census Bureau said on Wednesday that wholesale inventories edged up 0.1%, revised down from the 0.2% gain estimated last month.

Stocks at wholesalers rose 0.2% in July. Economists polled by Reuters had expected that the rise in inventories, a key part of gross domestic product, would be unrevised at 0.2%.

Inventories climbed 0.6% on a year-on-year basis in August.

Private inventory investment contributed to the economy's 3.0% annualized growth rate in the second quarter. Inventories and trade are the most volatile components of GDP.

The government reported on Tuesday that imports fell in August, helping to compress the trade deficit that month.

Trade is likely to be neutral on economic growth in the third quarter after being a drag for two straight quarters. Retail inventory data next week could shed more light on GDP growth estimates, currently converging around a 3.2% pace.

Wholesale motor vehicle inventories gained 0.1% after accelerating 1.4% in July. Excluding autos, wholesale inventories nudged up 0.1% in August. This component goes into the calculation of GDP.

Sales at wholesalers dipped 0.1% in August after surging 1.1% in July. At August's sales pace it would take wholesalers 1.35 months to clear shelves, unchanged from July.

(Reporting by Lucia Mutikani; Editing by Andrea Ricci)