Moss, who cut a STG3 million ($A7.15 million) deal with Topshop, is the latest celebrity hired by a retailer to help win an increasingly competitive battle for fashion-savvy mid-market shoppers.

Madonna is now starring at H&M and will soon be followed at the Swedish chain by Kylie Minogue, while up and coming singer Lily Allen has struck a deal with Britain's New Look.

Set up costs for the US stores will be in the region of $100 million ($A120 million) and Green said he was aiming for total space in Manhattan of 150,000 square feet across the three stores but wouldn't be limited by that.

"We will do something bigger if it's something fabulous," he said.

Topshop's launch in the United States is a bold move with rivals like Marks & Spencer having failed to break into the world's most competitive retail market in the past. Green did not rule out New York being the start of a bigger expansion.

"Let's get the New York stores open first and then we will see from there. It's a big country," he said.

While H&M saw its March sales surge 17 per cent thanks to Madonna, Moss, whose every fashion move is followed by the British press, was expected to be a bigger hit because of her history of inspiring millions of women's wardrobe choices.

The look she pulls together for herself - skinny jeans and waistcoats, floral mini dresses and sawn off denim shorts - continues to drive global fashion trends.

Green declined to give early sales figures for Moss' collection after its launch in 20 countries in the past two weeks following blanket coverage in the British press since it was announced last September.

However, he said more stock was being shipped to Barney's as it had nearly sold out of Moss' 50 piece collection in 19 stores nationwide. The range stretches from 12 pounds STG12 ($A28.62) for a tight-fitting vest to a leather jacket for 150 pounds ($A357.74).

"We are currently hoping to ship more merchandise to the States next week. Barney's is looking for more merchandise - they are chasing for more product," Green said.