Strong festive sales in the UK and continental Europe helped online fashion retailer Asos to report a good Christmas, but its shares fell.Asos said retail sales in the four months to December 31st rose 38% to £335.7m, with UK sales up 37% and international sales rising 38%, modestly ahead of analysts' forecasts.Continental European retail sales lifted 69% to £87.3m and rest-of-world sales were 19% up at £82.4m. US retail sales increased 28% to £32.3m.The shares dropped 435p to 6,425p just after midday in London.Broker Jefferies described the revenue rise as robust but said the rest-of-world performance was one possible weak spot.Freddie George at Cantor Fitzgerald said the company's move into China was likely to be a long term project.George said: "We believe the company will focus on a limited number of markets with a view to making them as significant as the UK rather than taking a scatter gun approach to global expansion."Clive Black at Shore Capital said Asos was a great company and was "set to remain hot property", but added: "The valuation of Asos is somewhat stratospheric, at a level at which we have some discomfort."Asos said the UK was becoming less important to the group's sales mix, with international retail sales now making up 60% of the total.The number of active customers rose 41% year-on-year to 7.9m. It said it had a strong balance sheet and cash position.Chief Executive Nick Robertson said improved delivery options, extra payment methods and the roll-out of its premier service in key international markets drove the performance.He said the group was increasing space at its Barnsley warehouse and it planned to open its central European distribution hub later this year."We have enjoyed another strong Christmas and made a good start to the financial year," he said. "The business continues to trade in line with expectations".PW