ASOS sales growth rebounds in US, Europe

* UK sales up 25 pct, international up 20 pct

* ASOS (LSE: ASC.L - news) helped by country by country pricing

* Firm recovering from tough 2014

* Shares (Berlin: DI6.BE - news) down 2.1 pct, rival Zalando (Swiss: OXZALG.SW - news) falls more (Adds details)

Jan 14 (Reuters) - British online fashion retailer ASOS reported a recovery in sales growth in the United States and Europe on Thursday, helped by moves to make its prices more responsive to exchange rate volatility, but it also saw its margins slip.

Established in 2000 for fashion-conscious 20-somethings, ASOS was an early ecommerce success story, but it is seeing growing competition from the likes of Germany's Zalando and British rival Boohoo.com, as well as from big chains moving online.

ASOS is investing in distribution, with construction just started on a giant new warehouse outside Berlin as it encroaches on Zalando's home territory, but spending on new facilities and technology, and price cuts, are hurting margins.

The retailer, which is recovering from a tough 2014 when it issued three profit warnings and suffered a warehouse fire, said its total retail sales rose 22 percent in September-December, the first four months of its 2015/16 fiscal year, but its retail gross margin fell by about 40 basis points, after slipping 10 bps in the year through August.

Its volatile shares, which had jumped this week on expectations of a stellar performance after Boohoo posted strong results, were down 2.1 percent at 0943 GMT.

"After a strong performance from Boohoo on Tuesday, much was expected from ASOS," said Jefferies analyst David Reynolds, describing the ASOS results as "solid" but noting that Zalando has the upper hand in scale, imperative for ecommerce success.

ASOS Chief Executive Nick Beighton, who took over last year from founder Nick Robertson, has said he wants ASOS to concentrate its efforts and investment on its main markets of Britain, France, Germany and the United States.

Faced with slowing growth abroad due to the strength of sterling, ASOS has introduced a system that allows it to charge prices country by country to reflect fluctuating exchange rates.

International sales jumped 20 percent in September-December, it said - after growing just 11 percent in the year ended August 2015 - driven by a 42 percent surge in sales in the United States and a 29 percent rise in sales in the European Union. Sales to the rest of the world, however, including Australia, Russia and China, rose just 2 percent.

Beighton told a conference call for journalists that the weather had made the period very difficult to predict but said the retailer had still seen record sales around the Black Friday discount weekend in November.

Zalando, which reports fourth-quarter results on Jan. 19, has seen its shares fall since it wrote to analysts on Wednesday to say it could not completely decouple itself from a weak fashion sector hit by unseasonably mild weather.

Its shares were down a further 4.5 percent on Thursday.

ASOS said its average order frequency and average basket value were up in the four months through December and active customers increased 18 percent on the year to 10.7 million.

Sales in Britain rose 25 percent in September-December, after a 27 percent rise in the 2014/15 fiscal year. (Reporting by Emma Thomasson; Editing by Susan Thomas and Susan Fenton)