Aspiring U.S. homebuyers rushed to take advantage of a tax credit for first-time owners that expires in November, driving up the number of signed sales contracts for the seventh straight month in August.

The National Association of Realtors said Thursday its index of sales agreements rose 6.4 percent from July to 103.8, beating forecasts. It was the highest since March 2007 and 12 percent above a year ago. Economists surveyed by Thomson Reuters expected the index would rise to 98.6.

Typically there is a one- to two-month lag between a contract and a done deal, so the index is a barometer of future sales. However, new rules for home appraisals and rigid lending standards have scuttled many sales agreements recently. In addition, the index may also double-count some buyers who agree to purchase other homes after the first deal falls through.

These factors have made the index a less reliable gauge for completed sales. Despite a steady increase in the number of signed contracts this summer, for example, completed sales actually took an unexpected 2.7 percent dip in August.

“Perhaps the real question is how many transactions are being delayed in the pipeline, and how many are being canceled,’’ Lawrence Yun, the Realtors’ chief economist, said in a statement. “Without historic precedents, it’s challenging to assess.’’

Pending sales were up 16 percent in the West and 8 percent in the Northeast. They were up 3 percent in the Midwest and nearly 1 percent in the South.

The U.S. housing market has been rebounding from the worst downturn in decades. But it’s uncertain how long that will last, especially if Congress does not extend the first-time homebuyers tax credit of up to $8,000 that expires Nov. 30.

Prices in most of the U.S. have finally started to rise from the depths of the housing slump. The Standard & Poor’s/Case-Shiller home price index of 20 major cities rose 1.2 percent in July from a month earlier, according to seasonally adjusted data released Tuesday. Prices, however, are still more than 13 percent below last year.

Housing experts, however, are divided on whether the price gains signal a definite bottom to the worst housing downturn in decades or just a brief respite from plummeting prices.