Contracts to purchase previously owned U.S. homes fell by the most in more than three years in September - in a sign the softer economy and a rise in mortgage rates are hurting the housing market, says the National Association of Realtors.

The NAR's Pending Homes Sales Index, based on contracts signed last month, plunged 5.6 percent; the fourth monthly fall in a row. Contracts fell across all four major regions.

It was the largest one-month drop since May 2010, when a home-buying tax credit was expiring, bringing the index to its lowest since December.

The industry trade group says the drop hints that existing home sales will be down this quarter and flat going into next year.