STOCKHOLM Aug 15 (Reuters) - Swedish fashion firm Hennes & Mauritz said on Friday its sales jumped a higher-than expected 17 percent in July from a year ago in local currencies as shoppers hit the stores for summer wear.

That beat a mean forecast in a Reuters poll of analysts for 11 percent and was above the 12 percent increase it had in June.

Revenues at the Swedish budget fashion retailer have been buoyed by brisk spring and summer sales, helping the firm post its tenth straight month of double-digit growth.

July is the second month of H&M's fiscal third quarter.

H&M says its market share has risen despite tougher competition from discount rivals such as Britain's Primark , U.S. group Forever 21 and online firms ASOS and Zalando.

It has invested heavily in its online business and plans to launch websites in eight to 10 markets in 2015 after rolling them out in Spain, Italy and China later this year.

It has also been busy broadening its product offering with brands such as COS, & Other Stories and H&M Home.

H&M, the world's second-biggest fashion retailer after Zara-owner Inditex, said it had a total of 3,314 stores at the end of July against 2,940 a year earlier, up 12.7 percent.

Still, analysts have been cautious about the outlook for the second half of the year, saying margins could come under pressure due to competition and rising wages in Asia where the retailer produces the majority of its garments.

H&M itself has been careful to be too optimistic over sales in Europe, where economic recovery still looks patchy.

And while strong growth in the United States is expected in the third quarter, the pace has likely slowed a bit with data this week showing a surprise weakness in retail sales, a sign that stagnant incomes remain a constraint.

On Thursday, retail giant Walmart Stores Inc reported flat U.S. same-store sales for the second quarter and cut its full-year profit forecast. Department store operator Kohl's Corp said its quarterly same-store sales fell 1.3 percent.

The Swedish fashion firm's share price was down 2.8 percent year-to-date at the close of trade on Thursday compared with a 10.3 percent fall in rival Inditex. (Reporting by Mia Shanley, editing by Anna Ringstrom and William Hardy)