Long-term U.S. mortgage rates declined this week after rising for three straight weeks, continuing to lure prospective homebuyers.

Mortgage giant Freddie Mac said Thursday the average for the benchmark 30-year fixed-rate mortgage fell to 3.43 percent from 3.48 percent last week. The average rate is down sharply from 3.91 percent a year ago.

The 15-year fixed mortgage rate dipped to 2.74 percent from 2.78 percent last week.

Record-low interest rates this year have helped spur home purchases and boost the housing market. The Federal Reserve has been holding its key short-term rate near zero since 2008, and a statement from the Fed last week after its latest policy meeting had led many economists to conclude that a strengthening economy would lead the central bank to resume raising rates as soon as September.

But after a government report last Friday showed a surprisingly lackluster economy in the second quarter, many economists said a September rate hike was now probably off the table. The Commerce Department data showed that gross domestic product — the broadest gauge of the economy — grew just 1.2 percent in the April-June period.

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 2.73 percent, down from 2.78 percent last week. The fee held at 0.5 point.