Stacks of rice at a supermarket in China. - AP

The two biggest US warehouse retail chains are limiting how much rice customers can buy because of what Sam's Club, a division of Wal-Mart Stores Inc, called "recent supply and demand trends."

The broader chain of Wal-Mart stores has no plans to limit food purchases, however.

The move comes as US rice futures hit a record high amid global food inflation, although one rice expert said the warehouse chains may be reacting less to any shortages than to stockpiling by restaurants and small stores.

Sam's Club followed Seattle-based Costco Wholesale Corp, which put limits in at least some stores on bulk rice purchases.

Sam's Club declined to say if this is the first time it has restricted sales of bulk foods.

The limits affect 20-pound bags, not retail-sized portions.

Costco President and CEO Jim Sinegal declined to discuss the issue Wednesday with an AP reporter.

Limit for customers

Sam's Club said it will limit customers to four bags at a time of imported jasmine, basmati and long grain white rice.

The warehouse chain caters heavily to small businesses, including restaurants.

Sam's Club spokeswoman Kristy Reed said she could not comment on whether the problem was caused by short supplies or by customers stocking up in anticipation of higher prices.

USA Rice Federation spokesman David Coia said there is no rice shortage in the United States.

"It's possible that small restaurants and bodega-type neighbourhood stores may be purchasing rice in larger quantities than they do typically to avoid higher prices," Coia said about the warehouse chain restrictions.

The Sam's Club restriction is effective immediately at all locations where quantity restrictions are allowed by law.

Only on rice

It does not apply to other staples such as flour or oil.

"We are working with our suppliers to address this matter to ensure we are in stock, and we are asking for our members' cooperation and patience," Reed said in a statement.

Sam's Club has 593 stores compared with 2,523 Wal-Mart supercenters that combine a full grocery section with general merchandise.

Costco has 534 warehouses worldwide, most of them in the United States.

US rice futures soared to an all-time high Wednesday as investors bet that surging world demand will continue to pressure already dwindling stockpiles. Rice for the most actively traded July contract jumped 62 cents to US$24.82 per 100 pounds on the Chicago Board of Trade, after earlier rising to a record US$24.85.

Relentless demand from developing countries and poor crop yields have pushed rice prices up 70 per cent so far this year, raising concerns of severe shortages of the staple food consumed by almost half the world's population.