By Jessica Wohl

(Reuters) - Walmart has started to limit sales of ammunition to three boxes per customer per day due to limited supplies, a spokeswoman said on Wednesday.

Sales of guns and ammunition have risen across the United States since the December 14 shooting at a Newtown, Connecticut elementary school.

Walmart U.S., is the largest unit of Wal-Mart Stores Inc the world's largest retailer and the largest U.S. gun seller. The Bentonville, Arkansas-based retailer had said that it works to strike the right balance between serving hunters and sportsmen and ensuring that it sells firearms responsibly.

"In order to take care of as many customers as possible, starting Thursday, January 24, all ammunition sales were limited to three boxes per customer, per day as supply is limited at this time," spokeswoman Ashley Hardie told Reuters on Wednesday.

Walmart was a charter member of New York City Mayor Michael Bloomberg's coalition against illegal guns and adopted a 10-point code established by the Responsible Firearms Retailer Partnership that goes beyond what the law requires.

Data released on Monday by the Federal Bureau of Investigation's National Instant Criminal Background Check System showed that nine of the top 10 days for firearms checks from November 30, 1998, to January 27, 2013, including December 14, 2012, occurred since Adam Lanza killed 20 children and six adults at Sandy Hook Elementary School and then shot and killed himself. Lanza also killed his mother before the school massacre.

Walmart's policy change was reported earlier by The City Wire, an Arkansas news outlet. (http://www.thecitywire.com/node/26213#.UQmIBB1EF8E)

(Reporting by Jessica Wohl in Chicago; Editing by Leslie Gevirtz)