By Ellen Emmerentze Jervell And Sara Germano 

FÜRTH, Germany-- Adidas AG this week reported a surge in North American sales that top executives on Thursday said shows the German sportswear giant's troubled U.S. operations are finally improving.

Investors and industry insiders remain wary.

Adidas's North American sales rose by a currency-adjusted 7% in the quarter ended March 31, the company said Tuesday. The uptick in the world's biggest and most influential sports market came after Adidas significantly reworked its approach to the U.S. last year.

"Numbers don't lie," said Chief Executive Herbert Hainer at the company's annual general meeting here Thursday. "We are seeing the first successes. I think what we're doing is right."

For months, Adidas has been telling analysts that internal changes, including shifting top management and design resources to its North American headquarters in Portland, Oregon, would bring growth beginning early this year. It also said its margins in the region would be helped by a shift toward American sports, rather than soccer, and the launch of more America-targeted marketing campaigns.

"It's too early to see the first results of the 'Americanization strategy,'" said Cédric Rossi, an analyst at Bryan Garnier in Brussels. Adidas's North American sales growth, he said, was only "satisfactory" because Adidas reported "very weak numbers" in the region last year.

Adidas's position in the U.S. market has eroded in recent years. American consumers have seen the brand as off-mark, with the wrong products. Adidas last year fell to third place behind industry newcomer Under Armour Inc. and longtime rival Nike Inc. in U.S. retail sales of sports apparel and footwear, according to analysts Sterne Agee and SportScanInfo. Earlier this year, Adidas's North American president, Mark King, said the brand "in a very short time" would improve its cool factor.

At the annual meeting, Adidas's investors restrained their excitement.

Marco Scherer, a representative of Adidas shareholder Deutsche Asset Management, said the first quarterly results looked like "a good start," but asked whether Adidas could prove it wasn't "only running behind its competitors" in the North American market.

"Nothing takes as long as regaining trust," said Daniela Bergdolt, a lawyer at DSW, another company shareholder. Making Adidas attractive again in the U.S. will be "very expensive," she said.

Adidas's North American growth also came amid broader growth in the American sporting-goods market. Under Armour said its North American sales rose 20% in the first quarter and Nike said sales rose 6%.

Market-researcher Euromonitor said the U.S. sportswear market is expected to grow to $96.2 billion in 2015, up 6.6% from last year--and despite the fact that one in four Americans didn't exercise at all last year, according to a recent survey by the Physical Activity Council. Euromonitor hypothesized that so-called athleisure attire is reshaping the retail industry, with the boundary between fashion and sports "getting blurrier month by month."

Despite its reported positive revenue development, Adidas still lags behind the broader sportswear industry in terms of growth so far this year, said Neil Schwartz, vice president of business development for SportsOneSource. Overall, the U.S. retail sports footwear industry has grown 7% in unit-sales volume and 6% in dollars so far this year, according to the firm.

Driving that growth in the U.S. are sales of lower-priced fashion-athletic shoes, Mr. Schwartz said, a category that hasn't been Adidas's strength. Adidas said it is already working to correct that, and plans to roll out two new lines of running shoes priced under $100 within the next year.

Reebok, acquired by Adidas in 2006 for EUR3 billion ($3.4 billion), reported its eighth consecutive quarter of growth after many years of decline. The brand reported 9% sales growth during the first quarter.

Mr. Rossi, the Bryan Garnier analyst, said the impact of Adidas's recent moves in North America would only become evident in the second half of this year. Adidas's current U.S. strategy "is the right one," he said.

Matt Powell, an industry analyst at NPD Group, predicted "an improving climate" for the company in the U.S.

Write to Ellen Emmerentze Jervell at ellen.jervell@wsj.com and Sara Germano at sara.germano@wsj.com

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