WASHINGTON >> 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 said today that 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 sales and construction are up from a year ago, and prices are rising in most markets.

The number of Americans who signed contracts to buy homes rose in March to the highest level in three years, the National Association of Realtors said earlier this week.

And home prices rose 9.3 percent in February compared with a year earlier, according to the Standard & Poor’s/Case-Shiller 20-city home price index. That’s the biggest annual gain in nearly seven years.

Still, prices are surging because more buyers have fewer homes to bid on. And sales are rising in some markets hit hardest by the housing crisis because investors are scooping up homes before prices rise further.   

Mortgage rates are falling because they tend to track the yield on the 10-year Treasury note. The yield fell to 1.63 percent on Thursday — the lowest this year.

The yield declines when demand for Treasurys increases. The Federal Reserve has been buying Treasury bonds since the fall. And in recent weeks, concerns that economic growth is slowing in the U.S. and abroad has led investors to shift money into safer assets, like Treasurys, and away from stocks.

To calculate average mortgage rates, Freddie Mac surveys lenders across the country on Monday through Wednesday each week. The average doesn’t include extra fees, known as points, which most borrowers must pay to get the lowest rates. One point equals 1 percent of the loan amount.

The average fee for 30-year mortgages edged down to 0.7 point from 0.8 point last week. The fee for 15-year loans was unchanged at 0.7 point.

The average rate on a one-year adjustable-rate mortgage fell to 2.56 percent from 2.62 percent last week. The fee for one-year adjustable-rate loans held steady at 0.3 point.

The average rate on a five-year adjustable-rate mortgage declined to 2.56 percent from 2.58 percent. The fee was stable at 0.5 point.