WASHINGTON (Reuters) -U.S. business inventories were unexpectedly unchanged in August, with stocks at retailers falling amid strong sales.

The unchanged reading in inventories reported by the Commerce Department's Census Bureau on Tuesday followed a 0.1% gain in July. Inventories are a key component of gross domestic product and one of the most volatile. They increased 1.1% year-on-year in August. The report was delayed by the recently ended 43-day shutdown of the government. 

Business inventories decreased at a $18.3 billion annualized rate in the second quarter, subtracting 3.29 percentage points from GDP. That was, however, more than offset by a record 4.83 percentage point contribution from a smaller trade deficit.

The Atlanta Federal Reserve is forecasting gross domestic product grew at a 4.2% annualized rate in the third quarter. 

The government will release the third-quarter GDP estimate on December 23. The economy grew at a 3.8% pace in the April-June quarter.

Retail inventories fell 0.1% in August, instead of being unchanged as estimated in an advance report published in September. They rose 0.1% in July. 

Motor vehicle inventories fell 0.5%, rather than declining 0.4% as previously reported. They rose 0.2% in July.

Retail inventories excluding autos, which go into the calculation of GDP, were unchanged. They were previously reported to have increased 0.3%.

Wholesale inventories were flat in August as were stocks at manufacturers.

Business sales rose 0.2% in August after increasing 1.0% in July. Sales at retailers advanced 0.5%. At August's sales pace, it would take 1.37 months for businesses to clear shelves, unchanged from July.