WASHINGTON (AP) - The average U.S. rate on the 15-year fixed mortgage fell to a new record low last week, and the rate on the 30-year fixed loan declined. Cheaper mortgages have encouraged more home-buying and refinancing. Mortgage buyer Freddie Mac says the average rate for the 15-year fixed mortgage slipped to 2.56 percent. That's the lowest on records dating to 1991. It fell from 2.61 percent last week, the previous record. The rate on the 30-year loan declined to 3.35 percent from 3.40 percent last week. That's near the average rate of 3.31 percent reached in November, the lowest on records dating back to 1971. Low mortgage rates have helped sustain a housing recovery that began last year. Home prices and sales are rising, and construction is up.