NEW YORK (CNN/Money) - Both Wal-Mart Stores Inc. and Federated Department Stores Inc. Monday reported disappointing back-to-school sales at stores open for at least a year.

Bentonville, Ark.-based Wal-Mart, the world's largest retailer, said "same-store" sales are running at the low end of its forecast for a rise of 4-to-6 percent in August, with warm weather hurting back-to-school sales.

The data are for the period Aug. 3-16. The four-week August sales period runs from Aug. 3 to Aug. 30.

"Back to school is off to a slow start as warm weather has affected apparel sales," Wal-Mart said in a weekly sales update message.

Best-selling products have included food, pet supplies, automotive supplies and electronics, the retailer said.

Wal-Mart (WMT: Research, Estimates) shares fell 92 cents to $53.79 Friday.

Separately, Cincinnati-based Federated said "same-store" sales were running below company expectations in the first two weeks of August.

The parent of Macy's and Bloomingdale's had said last week that it expected same-store sales to be down 1 percent-to-3 percent in the four-week August sales period.

Federated (FD: Research, Estimates) shares rose 13 cents Friday to close at $37.13.  

-- from staff and wire reports