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Pending home sales slipped in August as fewer Americans signed contracts to purchase a house, the fourth decline in the past five months.

The National Association of Realtors said Thursday that its pending home sales index fell 1.8% last month to 104.2. This measure of contract signings has tumbled 2.3% in the past year, with the sharpest annual decline of 11.2% in the west where homes generally cost more.

The recent setbacks suggest that the combination of rising prices, higher mortgage rates and a limited number of sales listings are hurting affordability.

In August contract signings slipped on a monthly basis in the four major geographic regions: Northeast, Midwest, South and West.

Pending sales are a barometer of home purchases that are completed a month or two later. Over the past year, the completed sales of existing homes have tumbled 1.5%.

The costs pressures caused by rising prices had been minimized by historically low mortgage rates. But the average interest charged on a 30-year fixed-rate mortgage was 4.72%, the highest since April 2011, according to the mortgage buyer Freddie Mac.

Also read: U.S. housing less affordable as borrowing costs rise