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Shares in Asos have fallen despite the online fashion retailer posting a 10% rise in half-year profits to £29.9m.

The firm said retail sales jumped 27% to £1.13bn for the six months to 28 February compared with the same period in the previous year.

The shares fell as Asos said it would spend up to £250m on its operations.

After falling as much as 10%, the stock recovered ground to be down just 1.2% in afternoon trading at £69.48.

Asos shares have risen almost £15 over the past 12 months, with the company now valued at £5.8bn. Marks & Spencer, in contrast is worth just £4.3bn.

Asos has spent about £95m on its business so far this year.

Half that sum has gone on technology and transformation programmes, with the remainder on infrastructure across its supply chain and head office.

Asos chief executive Nick Beighton said the money was needed to keep upgrading its 200 localised websites, and incorporate more artificial intelligence into services like its recommendations engine and visual search.

"Our customer engagement is going from strength to strength and we've achieved more than a billion site visits for the first time," he said.

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The firm will also open its warehouse in Atlanta months earlier than planned in July.

Asos is aiming to almost quadruple sales to £4bn to make it the "world's number one destination for fashion-loving 20-somethings".

Nicholas Hyett at Hargreaves Lansdown said Asos had a "tendency to underestimate capital expenditure requirements by some tens of millions a year".

"That the rapid growth isn't delivering any meaningful margin benefits only adds to the frustration," he added.

However, analysts at RBC Capital Markets, were more bullish: "Management will continue to prioritise market share gain and reinvest operating leverage into enhancing the proposition, which we believe is the winning strategy."