WASHINGTON — Inventories in the United States rose at a slower pace in August, indicating that unexpected strength in sales may be starting to drain stockpiles.

The 0.6 percent increase in goods on hand followed a 0.8 percent gain in July, Commerce Department data showed Monday in Washington. Sales at factories, wholesalers, and retailers climbed 0.5 percent after advancing 0.9 percent the prior month.

The biggest back-to-back gains in retail sales in almost two years may be making it difficult to keep shelves and warehouses stocked as companies reined in spending ahead of looming year-end tax and government spending changes in the United States. The need to replenish depleted inventories may help give manufacturing a boost in the last three months of the year.

‘‘Businesses are operating in a fairly lean environment right now,’’ Carl Riccadonna, a senior US economist at Deutsche Bank Securities in New York, said before the report.

‘‘Definitely there’s hesitancy about these other economic issues that I think is weighing on inventory accumulation.’’

At the current sales pace, businesses had enough goods on hand to last 1.28 months, the same as in July.