WASHINGTON (AP) — The average rate on the 30-year fixed mortgage fell to a record 3.91 percent this week, the third time this year that rates have hit new lows.

Freddie Mac said Thursday that the average on the 30-year home loan fell from 3.94 percent the previous week. The 3.91 percent rate is the lowest average for long-term fixed mortgages on records dating to the 1950s

The average on the 15-year fixed mortgage was unchanged this week at 3.21 percent. That's also a record.

Low rates offer a historic opportunity for those who can afford to buy a home or refinance. But many Americans either can't take advantage of the rates or have already done so.

Rates have been below 5 percent for all but two weeks in 2011. Even so, this year is shaping up to be one of the worst ever for home sales.

Rates could fall further still. Many economists think the yield on the 10-year Treasury note could creep lower in 2012. Long-term mortgage rates tend to track the 10-year Treasury yield.

Should the Federal Reserve launch a new program of bond purchases in the coming months to try to help the economy, it could further drive down mortgage rates.

Frank Nothaft, Freddie Mac's chief economist, has said that despite the super-low loan rates, foreclosures and falling home values have created obstacles for would-be buyers.

But builders could see more interest from buyers in the coming months if mortgage rates stay low. The low rates contributed to a modest 2-point increase in builder sentiment in the latest National Association of Home Builders survey released this month, said Yelena Shulyatyeva, an analyst at BNP Paribas. Those rates, coupled with falling prices, could draw more people into the market, she said.

Sales of previously occupied homes are just slightly ahead of last year's dismal sales figures. New-home sales appear headed for their worst year on records going back half a century.

Mortgage applications fell about 2.6 percent last week, according to the Mortgage Bankers Association. Refinancing fell 1.6 percent. And loan applications to buy homes fell nearly 5 percent. Over the past four weeks, the level of mortgage applications has been relatively unchanged.

Some lenders have reported an increase in applications through the Obama administration's refinancing program. That program was broadened in October to allow up to 1 million more homeowners lower their mortgage payments. But the MBA said such government-assisted loans account for just a small portion of refinancing.

High unemployment and scant wage gains have made it harder for many people to qualify for loans. Many Americans don't want to sink money into a home that they fear could lose value over the next few years.

To calculate the average rates, Freddie Mac surveys lenders across the country Monday through Wednesday of each week. The average rates don'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 the 30-year loan fell to 0.7 from 0.8; the average on the 15-year fixed mortgage was unchanged at 0.8.

For the five-year adjustable loan, the average rate fell to 2.85 percent from 2.86 percent. The average on the one-year adjustable loan declined to 2.77 percent from 2.81 percent.

The average fees on the five- and one-year adjustable-rate loans were unchanged at 0.6.