Feb 10 - It may take Wal-Mart Stores Inc more time than it expected to increase sales at existing U.S. stores, adding pressure to the world's largest retailer and its stock price.

Shoppers leaving Walmart in Mexico City

In mid-November, Wal-Mart was giddy about its prospects. It said U.S. sales at its discount stores open at least a year should finally rise after six consecutive quarterly declines.

A strong showing early in the holiday season buoyed those expectations, as the retailer's return to selling a wide variety of discounted goods appeared to be paying off.

Then, several major snowstorms and frigid weather blanketed the country, forcing some shoppers to stay home. Competitors such as Target Corp said that those who made it to stores hunted for basics like food, bargains and little else, and gas prices rose, adding strain to those on tight budgets.

Now, as Wal-Mart prepares to report its quarterly results on Feb. 22, some question whether it can return to sales growth in its domestic discount chain, its largest business.

"Everybody is sensitive to the sales number," said Gilford Securities analyst Bernard Sosnick. "Whether or not it is caused by the weather, if it comes up short the stock is going to feel a setback."

Wal-Mart officially forecast fourth quarter same-store sales in the range of a 1 percent decline to a 2 percent rise. But back in November, then-CFO Tom Schoewe said that he was confident the company would report an increase.

Some analysts expect Wal-Mart to miss that goal. Among them is UBS analyst Neil Currie, who cut Wal-Mart to "neutral" from "buy," saying the sales recovery could take time.

Currie now expects that same-store sales at Wal-Mart's U.S. discount stores fell 0.8 percent, down from a prior forecast of a rise of 0.3 percent.

Analysts polled by Thomson Reuters, excluding UBS's Currie, call for sales to rise 0.5 percent to 2 percent, with an average target of a 0.8 percent increase as of Wednesday.

Such sales fell 2 percent a year earlier, and have fallen in smaller percentage terms in each quarter since then.

By Jessica Wohl