Wal-Mart Stores Inc. broke a two-year decline in quarterly same-store sales at its key U.S. operations, but the increase in sales came at a cost—lower margins.

And the retail giant's margins will continue to face pressure. Wal-Mart Chief Executive Mike Duke Tuesday told analysts the retailer is "investing in low prices for the holidays" in the U.S. The retailer also posted a 2.9% drop in its fiscal third-quarter earnings, citing, in part, higher costs.

Bill Simon, head of Wal-Mart's U.S. operation, said that grocery inflation was about 4% during the latest quarter, but "the impact to ...