WASHINGTON (AP) — 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 percent last month to 104.2. This measure of contract signings has tumbled 2.3 percent in the past year, with the sharpest annual decline of 11.2 percent 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 percent.

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 percent, the highest since April 2011, according to the mortgage buyer Freddie Mac.