WASHINGTON — A measure of Americans who signed contracts to buy homes fell last month after reaching a 2½-year high in November. Sales were held back by a limited supply of available homes.

The National Association of Realtors said Monday that its seasonally adjusted index for pending home sales dropped 4.3 percent in December from November to 101.7. That’s still 6.9 percent higher than it was a year ago.

The decline signals that sales of previously occupied homes may cool off in the coming months. There’s generally a one- to two-month lag between a signed contract and a completed sale.

Still, completed sales of previously occupied homes rose last year to their highest level in five years, one of many signs of recovery in the housing market last year. And the Realtors’ group forecasts that sales will rise 9 percent this year. Stable job gains and record-low mortgage rates have encouraged more people to buy homes.

A tight supply of homes for sale has pushed up prices and made builders more confident to step up construction.