WASHINGTON (AP) — The average rate on a 15-year fixed mortgage has fallen to its lowest level in decades.

The rate for the popular refinancing option dropped to 3.54 percent this week from 3.66 percent last week, Freddie Mac said Thursday. That's lowest result since the mortgage buyer began tracking it in 1991.

The average rate on the 30-year fixed loan fell to a yearly low of 4.39 percent from 4.55 percent the previous week.

Mortgage rates tend to track the yield on the 10-year Treasury note. A weakening U.S. economy has led many investors to shift money from stocks to bonds, which are seen as safer bets. That has pushed Treasury yields to their lowest level this year. Bond yields fall as demand increases.

Low mortgage rates and depressed home prices have done little to revive the moribund housing market.

Sales of previously occupied homes fell in June for a third straight month to a seasonally adjusted 4.77 million. The pace is lagging behind the 4.91 million homes sold last year — the fewest since 1997. In a healthy economy, people buy roughly 6 million homes per year.

New-home sales also declined in June and are trailing last year's sales, which were the worst on records dating back nearly half a century.

Many people can't take advantage of the low mortgage rates. Banks are insisting on higher credit scores and larger down payments. Some potential homebuyers are holding off because they believe prices have yet to bottom out.

High unemployment, millions of foreclosures and tighter credit are likely to keep people from buying homes in the second half of the year, economists say.

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

The average rate on a five-year adjustable-rate mortgage fell to 3.18 percent, its lowest level on records that go back to January 2005. Last week's reading of 3.25 percent also was a record low.

The average rate for one-year adjustable-rate loans rose to 3.02 percent from 2.95 percent last week. Last week's average rate matched the record low set two weeks earlier on records dating back to 1984.

The rates do not include extra fees known as points. One point is equal to 1 percent of the total loan amount.

The average fees for the 30-year and 15-year fixed loans and the 5-year adjustable loan were unchanged at 0.8 point, 0.7 point and 0.6 point, respectively. The average fee for the one-year ARM fell to 0.5 point from 0.6 point last week.