WASHINGTON (AP) - Average U.S. rates for fixed mortgages rose sharply last week, making home-buying slightly less affordable. Mortgage buyer Freddie Mac says the average rate on the 30-year loan jumped to 4.46 percent from 4.29 percent last week. The average on the 15-year fixed loan increased to 3.47 percent from 3.30 percent. Rates have risen a full percentage point since May after the Federal Reserve signaled it might slow its bond purchases by year's end. Rates peaked at 4.6 percent in August. Mortgage rates have stabilized since September, when the Fed surprised markets by taking no action. And rates remain low by historical standards. The Fed meets later this month and could slow the bond purchases if the economy shows further improvement. The bond purchases are designed to keep long-term rates low.