STOCKHOLM, Sept 30 (Reuters) - Hennes & Mauritz , the world's third-biggest clothing retailer by sales, reported lower-than-expected third-quarter pretax profits on Tuesday as sales slipped in tough market conditions.

The firm made a pretax profit of 4.59 billion crowns ($680 million) against a mean forecast of 5.03 billion in a Reuters poll of analysts and 4.40 billion in the same period the previous year.

Sales in August, the final month of its third quarter, were up 8 percent against an expected 13.4 percent rise, with turnover in stores open a year or more down 3 percent.

Analysts had expected August like-for-like sales to have risen 1.5 percent, according to the mean forecast. H&M only gives whole numbers for the percentage change in sales and does not provide a currency figure.