WASHINGTON (Reuters) - U.S. business inventories increased in August, lifted by stocks at retailers, government data showed on Thursday.

Inventories rose 0.3% after a similar gain in July, the Commerce Department's Census Bureau said. The increase in inventories, a key component of gross domestic product, was in line with economists' expectations.

Inventories advanced 2.4% 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 gross domestic product. August's trade data last week suggested that trade could be a small drag on economic growth in the third quarter.

Retail inventories increased 0.6% in August, revised up from the 0.5% estimated in an advance report published last month. They increased 0.8% in July. Motor vehicle inventories rose 0.8% as previously reported. They advanced 1.2% in July.

Retail inventories excluding autos, which go into the calculation of GDP, climbed 0.5% instead of 0.4% as reported last month. They gained 0.6% in July.

Wholesale inventories gained 0.1% in August and stocks at manufacturers also edged up 0.1%.

Business sales slipped 0.2% in August after rising 1.1% in July. At August's sales pace, it would take 1.38 months for businesses to clear shelves, up from 1.37 months in July.

(Reporting by Lucia Mutikani)