WASHINGTON – Average U.S. rates for 30-year and 15-year fixed mortgages fell to fresh record lows this week, offering more incentive for Americans to buy or refinance homes.

Mortgage buyer Freddie Mac said Thursday that the rate on the 30-year loan fell to 3.84 percent, the lowest since long-term mortgages began in the 1950s. That’s below the previous record rate of 3.87 percent reached in February.

The 15-year mortgage, a popular option for refinancing, dropped to 3.07 percent, also a record. The previous record of 3.11 percent was hit three weeks ago.

Cheaper mortgage rates haven’t done much to boost home sales. Rates have been below 4 percent for all but one week since early December. Yet sales of both previously occupied homes and new homes fell in March.

Analysts suspect some of that weakness reflected a warm winter, which pulled sales that would normally occur during the spring buying season into January and February.

Still, many potential buyers can’t qualify for loans or afford higher down payments required by banks. Home prices in many cities continue to fall, making those that can afford to buy uneasy about entering the market. And many who can afford to buy or refinance have already taken advantage of lower rates.

Mortgage rates are lower because they tend to track the yield on the 10-year Treasury note. Mixed news on the U.S. economy and Europe’s debt crisis have led investors to buy more Treasurys, which are considered safe investments. As demand for Treasurys increases, the yield falls.

To calculate the average rates, Freddie Mac surveys lenders across the country on Monday through Wednesday of each week.

The average rage does not 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 loans was 0.8 last week, up from 0.7 the previous week. The fee on 15-year loans was 0.7, the same as last week.

The average on one-year adjustable rate loans also dropped to a record low of 2.7 percent last week, down from 2.74 percent last week. The fee on one-year adjustable rate mortgages was 0.6, unchanged from last week.