The number of Americans who signed contracts to buy previously owned homes rose in December for the first time in four months, signaling that slumping prices may be boosting demand.

The index of pending home resales climbed 6.3%, to 87.7, the first increase since August, from a revised 82.5 in November, the National Association of Realtors said Tuesday. Pending sales rose in two of four regions.

"Lower prices probably have attracted some buyers," said David Sloan, a senior economist at 4Cast Inc. in New York, who projected an increase. Still, "the rise may be difficult to sustain."

Economists had forecast pending sales would be unchanged in December after an originally reported drop of 4% in November, according to the median forecast of 28 economists in a Bloomberg News survey. Estimates ranged from a drop of 5% to a 2% increase.

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