WASHINGTON — The average U.S. rate on a 30-year fixed mortgage declined for a third straight week to 5.71 percent as record oil prices lessened borrowing costs, according to Freddie Mac.

The rate decreased from 5.77 percent the prior week and was 5.77 percent a year ago, said Freddie Mac, the congressionally chartered provider of mortgage financing. The one-year adjustable rate decreased to 4.48 percent the week ending today, from 4.56 percent. The 15-year fixed rate fell to 5.32 percent from 5.35 percent.

Rising petroleum prices, spurred on by supply interruptions from Hurricane Katrina, have slowed economic growth and pushed mortgage rates down. Some economists are predicting that borrowing costs will continue to decline, even though Federal Reserve policymakers have been raising short-term lending rates and warning that rising home price gains are unsustainable.

"This oil-induced crisis from Katrina is going to be good for housing," said John Herrmann, director of economic commentary at Cantor Fitzgerald LP in New York. "Mortgage rates are headed lower and that's going to support the housing market. It will help with refinancing activity and construction."

Herrmann predicted that the average weekly rate for 30-year fixed mortgages would decline to 5.6 percent the next two weeks and could fall as low as 5.5 percent before stabilizing.

The average monthly payment on a $100,000 mortgage at this week's average 30-year fixed lending rate of 5.71 percent was $581, compared with $550 when rates reached a four-decade low of 5.21 percent in June 2003.

Higher oil prices have contributed to lowering bond yields, which many mortgage lenders use to set home borrowing costs. Oil prices on the New York Mercantile Exchange have risen 12 percent in the past month, reaching $68.95 a barrel at midday. Yields on the 10-year government note have narrowed to 4.038 percent from 4.312 percent during the same period, while the average weekly rate on a 30-year mortgage has fallen to 5.71 percent from 5.82 percent.

Lower borrowing costs may bring some prospective home buyers back into the market who had been pushed out by rising prices.

"With the hurricane pushing energy prices up, markets have cut their view of what the Fed's going to do over the next couple quarters," said Wesley Beal, chief U.S. economist at IDEAglobal.com in New York. "It's a positive for housing because it's stopped the upward march of mortgage rates at least for now."

Before Katrina, most economists had expected home borrowing costs to increase steadily in response to the rising federal funds target rate. Fed policymakers have raised the nation's benchmark short-term lending rate from 1 percent to 3.5 percent since June 30 of last year.

Historically low borrowing costs have spurred home sales and supported construction spending, which totaled $617.9 billion in the first seven months of this year. That's 9.3 percent more than the same period last year, according to a report today from the Commerce Department.