CHICAGO (Reuters) - Wal-Mart Stores Inc. raised its forecast Monday for August sales gains at its U.S. stores open at least a year to a range of 4 to 6 percent after back-to-school demand proved stronger than initially expected.

Wal-Mart (WMT: Research, Estimates) shares rose 59 cents to $58.99 in before-hours trading Monday.

On a recorded message updating sales in the week ended Aug. 22, Wal-Mart said food, electronics, men's clothing, hardware, pharmaceuticals, toys and intimate apparel were among the best-selling categories.

The Bentonville, Ark.-based retailer previously expected a 3 to 5 percent increase in August same-store sales. But it said last week that sales may beat its forecast, helped by strong back-to-school spending.