Freddie Mac says the average interest rate on a 30-year fixed mortgage fell to 4.01% this week. That's the lowest rate since 1951.

The average rate on a 15-year fixed mortgage ticked down to 3.28%. Economists say that's the lowest ever for that loan.

Mortgage rates tend to track the yield on 10-year Treasury notes. Rates could fall further after the Federal Reserve announced last week that it would try to push long-term rates down further.

But low rates have so far done little to boost home sales or refinancing.

A second report Thursday said the number of Americans who signed contracts to buy homes fell in August, after a weaker-than-expected peak buying season.

The National Association of Realtors says its index of sales agreements fell 1.2% last month to 88.6.

A reading of 100 is considered healthy. The last time the index reached that level was in April 2010, final month that buyers could qualify for a federal tax credit that has since expired.

The number of people signing home contracts rose in both May and June. But those increases didn't make up for a huge drop in April, when signings fell more than 11%. Over the past two months, signings have declined 2.5%.

Contract signings fell across most of the country. July's index fell 5.8% in the Northeast, 3.7% in the Midwest and 2.4% in the West. It rose 2.6% in the South.

Contract signings are usually a reliable indicator of where the housing market is headed. There's typically a one- to two-month lag between a contract and a completed deal.

But the Realtors group says a growing number of buyers have canceled contracts after appraisals showed the homes were worth less than the buyers had bid. A sale isn't final until a mortgage is closed.

Home loans are also harder to come by. Many lenders are requiring 20% down payments and strong credit scores to qualify.

The pace of sales for previously occupied homes is slightly above last year's 4.91 million sold, which was the fewest since 1997. In a healthy economy, Americans would buy roughly 6 million homes each year.

In August, sales of new homes fell for a fourth straight month. This year is shaping up to be the worst for new-home sales on records dating to 1963.

Even so, homes are the most affordable they've been in decades. Prices in some metro areas have been cut in half. Still, sales in most areas remain weak.