Costco Wholesale Corp. said its comparable sales for the month of March increased 13% from last year, benefited from inflation in gasoline prices and strengthening foreign currencies.

Comparable sales from the U.S. for the five-week period ended April 3, including Mexico operations, increased 11 percent from last year and International sales were up 17 percent from a year ago.

The company said the five-week period included sales from its Mexico joint venture, as it began consolidating its Mexico operations on a prospective basis beginning with its 2011 fiscal year. Net sales for the month of March grew 17 percent to $8.33 billion from $7.15 billion in the same month last year. Excluding Mexico joint venture, the net sales increase would have been 14 percent, the company stated.

The five-week period for March included 35 days of sales compared to 34 days in the prior year. Easter falls in the reporting month of April this year. Costco said that the calendar shift positively impacted this year's March total and comparable sales by about one to two percent, benefited by one additional sales day this year, partially offset by several days of strong pre-Easter sales in the previous year.

Costco currently operates 580 warehouses, including 424 in the US and Puerto Rico, 80 in Canada, 22 in the UK, seven in Korea, six in Taiwan, eight in Japan, one in Australia and 32 in Mexico.

In addition, the company said it plans to open up to fourteen additional new warehouses prior to the end of its 2011 fiscal year ending August 28.