Online fashion retailer Asos saw its shares drop lower on Tuesday after reporting "challenging" trading conditions in the international markets during the first quarter of its financial year.However, the slowdown in sales during September and October eased in the final weekend of November, which was their "biggest ever trading week".The retailer said its UK retail sales grew 24% to £104.8m in the three months to November, but international retail sales only rose 2% to £141.5m.Asos reported total retail sales growth of 8%, slower than forecasts of 9% and much worst than the 38% growth seen at the same time last year.The weaker results were due to challenging international trading conditions, especially in the European market, strong pound and a fire at its Barnsley warehouse.Chief executive Nick Robertson said: "September and October were challenging as we completed the automation programme in Barnsley in time for peak."Our sales have since gathered momentum and we had our biggest ever trading week over cyber weekend in November."He continued: "During the quarter, we received £6.3 million of insurance proceeds related to the fire at Barnsley in June, which we are deploying to accelerate our investment in international pricing. After taking this into account, we expect full year profits to be in line with expectations."Numis analysts commented: "We remain confident that Asos has a unique proposition and believe that this can support sustainable long term global growth."The broker gave a 'buy' recommendation and a 3,250p target price.Shares were down 6% to 2,235p at 09:44 on Tuesday.