LONDON (Reuters) -Swedish fast-fashion retailer H&M reported weaker than expected sales for its first quarter on Thursday and said sales were up 1% so far in March, in a sign of a slow start to its spring and summer season.

H&M reported sales of 55.3 billion Swedish crowns ($5.52 billion) for the December to February quarter, missing analysts' mean estimate of 55.9 billion Swedish crowns.

"Our sales and earnings in the quarter were somewhat weaker than planned - but the first quarter is the smallest quarter of the year for us in terms of sales and margin, and we are confident going forward," CEO Daniel Erver said in a statement.

Increased discounting and marketing investments impacted H&M's profitability in the quarter, the company said, with the operating profit margin falling to 2.2% from 3.9% in the same period a year ago.

Erver, leading H&M for just over a year, is trying to turn its fortunes around and has ramped up marketing, spending on pop stars like Charli XCX to model its collections as he tries to make the brand more desirable and better compete against Zara and Shein.

($1 = 10.0259 Swedish crowns)