WASHINGTON (AP) — Long-term U.S. mortgage rates edged lower this week, remaining at historically low levels as a potential spur to home purchases.

Mortgage giant Freddie Mac said Thursday the average for the benchmark 30-year fixed-rate mortgage slipped to 3.43 percent from 3.45 percent last week. The average rate is down from 3.93 percent a year ago, and is close to its all-time low of 3.31 percent in November 2012.

The 15-year fixed mortgage rate declined to 2.74 percent from 2.76 percent last week.

Record-low interest rates this year have helped spark homebuying and boost the housing market.

Industry data released Monday showed that U.S. homebuilders are feeling more optimistic about the market this month, reflecting strong growth in new-home sales and prices. The National Association of Home Builders/Wells Fargo builder sentiment index rose 2 points to 60 following a downwardly revised reading of 58 in July.

To calculate average mortgage rates, Freddie Mac surveys lenders across the country at the beginning of 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 a 30-year mortgage remained at 0.5 point this week. The fee for a 15-year loan also was unchanged from last week at 0.5 point.

Rates on adjustable five-year mortgages averaged 2.76 percent, up from 2.74 percent last week. The fee fell to 0.4 point from 0.5 point.