Costco Wholesale Corp. (COST) on Thursday reported a 9 percent increase in June sales at its stores open at least a year, helped by international results and the timing of the July 4 weekend.

The largest U.S. warehouse club (search) operator said total sales for the five weeks ended on July 3 rose 12 percent to $5.22 billion.

The sales period covered 34 days this year compared with 33 days last year because of the timing of the July 4 holiday. Costco estimated that the extra sales day added 2 percent to 3 percent to its June sales.

Same-store sales were up 8 percent in the United States and 14 percent at international stores, helped by a strong Canadian dollar. Stripping out the currency effect, international sales would have been up just 7 percent, Costco said on a recorded message detailing its June sales.

Gasoline demand also boosted sales. Costco said the average price per gallon was $2.14 in June, up from $1.97 a year earlier. The retailer estimated that gasoline sales added 50 basis points to its June same-store tally.