Wal-Mart Stores Inc. (WMT), the world's biggest retailer, said on Monday its December sales at stores open at least a year were running at the low end of its expectations, but it was still counting on last-minute holiday shoppers to take up the slack.

The retailer said same-store sales at its Wal-Mart discount stores were running slightly below its forecast for the December period, which ends on Jan. 3, and its Sam's Club warehouse stores may report lower December same-store sales.

Wal-Mart had forecast December same-store sales would be up 3 percent to 5 percent.

In a recorded sales update, Wal-Mart said sales of seasonal merchandise were strong last week but it was difficult to predict sales in the holiday season as customers buy "closer to the event" of Christmas.

The sales update covered results through Friday, Dec. 20. The Saturday before Christmas is traditionally the busiest shopping day of the year.