WASHINGTON, Sept 17 (Reuters) - U.S. retail sales unexpectedly rose in August as a decline in receipts at auto dealerships was more than offset by strength in online purchases, suggesting that the economy remained on solid footing through much of the third quarter.

Americans spent a bit more at retailers last month, providing a small boost to the economy just as the Federal Reserve considers how much to cut its key interest rate.

A strong report could pave the way for a more modest rate cut, while a softer reading may spur the central bank into taking more drastic action.