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.

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.

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.