The weak economy has been a source of strength for Wal-Mart Stores, but at some point, the slowdown may start to drag.

The world's largest retailer reported an impressive 6.9 percent gain in first-quarter earnings, as its price-cutting attracted consumers trying to tighten their belts amid worries about their jobs, gasoline prices, and housing. Net sales rose 10.2 percent, to $94.1 billion.

But Wal-Mart also cautioned that the economy presented uncertainties for the rest of the year, saying that it expects sales at U.S. stores open at least a year to be essentially flat. The retailer forecasts earnings per share of 78 cents to 81 cents per share: the low end of analysts' estimates.

Last week, the company noted that it was seeing signs that U.S. sales would drop toward the end of the month, as shoppers awaited their next paycheck.

For the first quarter, Wal-Mart earned $3.02 billion, or 76 cents per share, compared with $2.83 billion, or 68 cents per share, in the quarter a year ago. International sales surged 22 percent, to $23.9 billion. Sales at U.S. stores open at least a year rose 2.9 percent, excluding fuel and gasoline sales.

"Our business is even more relevant to our customers today, given the current economic pressures," Wal-Mart's chief executive, Lee Scott, said on a conference call. "We're off to a solid start."

Douglas McIntyre on the 24/7 Wall St. blog says that Wal-Mart is achieving its growth at the expense of others. "There is no way that Wal-Mart could post numbers of this magnitude without taking business from other retailers," he says.