WASHINGTON (Reuters) - U.S. business inventories increased less than expected in September as a rise in stocks at retailers was partially offset by declines at manufacturers and wholesalers.

Inventories edged up 0.1% after advancing 0.3% in September, the Commerce Department's Census Bureau said in Friday. Economists polled by Reuters had forecast inventories, a key component of gross domestic product, gaining 0.2%.

Inventories rose 2.2% on a year-on-year basis in September. Inventories and trade are the most volatile components of gross domestic product. Private inventory investment was a small drag on GDP in the third quarter. The economy grew at a 2.8% annualized rate in the July-September quarter.

Retail inventories increased 0.9% in September, slightly revised up from the 0.8% estimated in an advance report published last month. They gained 0.8% in August.

Motor vehicle inventories accelerated 2.1% as previously reported. They rose 1.2% in August.

Retail inventories excluding autos, which go into the calculation of GDP, gained 0.2% instead of 0.1% as reported last month. They increased 0.6% in August.

Wholesale inventories fell 0.2% in September as did stocks at manufacturers.

Business sales increased 0.3% in September after slipping 0.2% in August. At September's sales pace, it would take 1.38 months for businesses to clear shelves, unchanged from August.

(Reporting by Lucia Mutikani)