Wal-Mart Stores Inc. (WMT) joined the chorus of retailers saying that economic conditions in the U.S. are making shoppers reluctant to spend.

The nation's biggest retailer said the reticence is striking hard at its low-income consumer base after mid-range retailers like Macy's Inc. (M) and J.C. Penney Co. (JCP) said last week that their own customers are being affected.

"Our core customer continues to be strained," Chief Financial Officer Charles Holley said in a call with reporters. And while not necessarily big stock market investors, Wal-Mart customers are feeling some confidence sapped by the U.S. debt downgrade and the market's wild ride. "The volatility in the headlines doesn't help the customer," he said.

Wal-Mart is in fact seeing more customers on public assistance. "They are really stretched right now," Holley said.

The situation puts Wal-Mart in a difficult spot, especially since it said for the first time on Tuesday that it has seen competitors narrowing their prices against its own. Being seen as providing less bargains erodes Wal-Mart's credibility as the "low cost leader," which could have reputational and business ramifications for the nation's biggest retailer. "We're not as low as we used to be," Holley said. The company plans steps like improving its supply chain and being more productive to help bring down prices, he said.

The problem is at U.S. Wal-Mart stores -- the company's biggest unit -- where sales at stores open more than a year fell 0.9% in the second quarter, their ninth straight drop and on the low end of guidance for a 1% drop to a 1% rise. For the third quarter, Wal-Mart expects same-store sales to be down by 1% to up by 1%. But the company is "committed to having a positive comp by the end of the year," Holley said.

"Wal-Mart U.S. came in a little light and still has work to do," said John Tomlinson, a director at ITG Investment Research. "They have indicated the price gap between themselves and competitors has narrowed, meaning it is not as advantageous for consumers to shop at Wal-Mart. Given what Wal-Mart stands for, it is imperative they have lower prices."

Admittedly, some of the siphoning of U.S. sales was a result of the retailer's own operational missteps, like removing sought-after merchandise from shelves. But while the company has made great strides in righting those mistakes, customers still are a bit reticent, because of the soft economy and apparent trips to rivals like dollar stores.

During the quarter, the average amount bought at Wal-Mart U.S. stores was up and customer traffic was down compared with a year ago. Both traffic and average amount bought, on year-over-year basis, improved sequentially each month in the period, the company said. U.S. sales rose 1.8% for the quarter.

Wal-Mart U.S. Chief Bill Simon said in a pre-recorded call for analysts that the company was "encouraged by the sales momentum we have from the second quarter" and said that July same-store sales were, in fact, up.

Grocery and health and wellness business, representing two-thirds of sales, continued to deliver positive comps. The hardlines and apparel businesses, which have had a tougher time, "are improving," Simon said.

Wal-Mart's international operations fared well, with sales on a constant currency basis up 7.1% for the second quarter. Mexico, the U.K., Canada, Brazil and China had the largest sales increases during the quarter, the company said.

Sam's Club, the retailer's wholesale club chain, remained a standout, with sales rising 5% on a same-store basis and comparable-store traffic and ticket, excluding fuel, both higher than last year, allowing the business to deliver comparable-store increases in nearly all categories, Wal-Mart said.

The quarter was a busy one for Wal-Mart, with the retailer taking steps to improve every facet of its business. The company-opened Wal-Mart Express, its first small concept stores in the U.S., closed on the purchase of a majority stake in South African retail chain Massmart and planned the revamping of its online business, an effort that was completed in the current quarter.

The measures are meant to boost every facet of Wal-Mart's business. Wal-Mart is also said to be exploring a potential acquisition of the Brazilian unit of French retailer Carrefour SA (CRRFY, CA.FR), two years after a previous attempt to do so ended in a disagreement over price.

The company raised its full-year earnings view to $4.41 to $4.51 a share from $4.35 to $4.50 and forecast current-quarter earnings of 95 cents to $1 a share while analysts surveyed by Thomson Reuters expect 97 cents.

For the quarter ended July 31, Wal-Mart reported a profit of $3.8 billion, or $1.09 a share, up from $3.6 billion, or 97 cents a share, a year earlier. It had projected earnings from continuing operations of $1.05 to $1.10 in May.

Revenue rose 5.4% to $109.37 billion, topping the $108.36 billion expected by analysts polled by Thomson Reuters.

Operating margin fell to 5.8% from 6%.

Wal-Mart shares were up 3.1% to $51.54.

-By Karen Talley, Dow Jones Newswires; 212-416-2196; karen.talley@dowjones.com