U.S. business inventories rose slightly more than expected in September as sales climbed to their highest level since October 2008, a government report showed on Monday, suggesting an upward adjustment to the third-quarter growth estimate.

The Commerce Department said inventories rose 0.9 percent to $1.40 trillion, the highest level since March 2009, after increasing by a revised 0.9 percent in August.

Markets had expected September inventories to rise 0.8 percent from a previously reported 0.6 percent increase in August.

Inventories are a key component of gross domestic product changes over the business cycle and have been a key driver of growth as the economy recovers from the worst downturn since the Great Depression.

September's larger-than-expected increase in inventories and August's upward revision suggest the government might raise its preliminary GDP growth estimate when it publishes its first revision this month. Initial estimates put third-quarter GDP at a 2.0 percent annual rate.

Business sales increased 0.5 percent to $1.10 trillion in September after rising 0.3 percent in August.

That left the inventory-to-sales-ratio, which measures how long it would take to clear shelves at the current sales pace unchanged at 1.27 months.