U.S. chain store sales fell in August as concerns about the strength of the economy drove consumers to cut back on spending, a report showed on Wednesday.

Instinet's Redbook Retail Sales Average slipped 1.7 percent in the four weeks ended Aug. 31 compared with the same period in the previous month.

"Back-to-school sales have not been meeting expectations, especially in the apparel and accessories merchandise classifications," the report said. "Most stores missed or appeared at the lower end of their targets."

The Redbook Average is compiled from a sample of general merchandise retailers representing about 9,000 stores. Same-store sales measure revenue at stores open at least a year.

The index is released weekly by Instinet Research, a division of Instinet, a Reuters-owned electronic brokerage.