WASHINGTON - The number of Americans who signed contracts to buy houses in November rose to the highest level in a year and a half, the best reading on pending home sales since the federal home-buying tax credit expired.

Still, the National Association of Realtors cautioned that a growing number of buyers were canceling their contracts at the last minute, making the gauge less reliable.

The Realtors group said Thursday that its index of sales agreements jumped 7.3 percent last month to a reading of 100.1. A reading of 100 is considered healthy. The last time the index was that high was in April 2010, one month before the tax credit expired.

Contract signings usually indicate where the housing market is headed. There is typically a one- to two-month lag between a signed contract and a completed deal.

But a sale isn't final until a mortgage is closed, and many sales are falling apart before that happens. One-third of Realtors say they had at least one contract scuttled in November and October, according to the Realtors' group. That's up from 18 percent in September.

Pierre Ellis, an analyst at Decision Economics, said potential buyers were "feeling comfortable with their personal situations and with the house-price trend." But the demand for homes is running into significant obstacles, he said, including tougher lending standards and bigger required down payments.

Homes are the most affordable they have been in decades. Long-term mortgage rates are at historic lows, and prices in most metro areas have tumbled since late 2006.

Yet this year will likely be the worst year for sales of newly constructed homes on records dating back to 1963. Sales of previously occupied homes are just barely ahead of 2008's dismal figures - the worst yearly showing since 1997.

Americans are holding off on buying homes for a number of reasons. High unemployment and weak job growth have deterred many potential buyers. Loans are harder to come by. Lenders are requiring bigger down payments and strong credit scores to qualify.

Even those with good credit and stable finances are hesitant to buy out of concern home prices will keep falling.