Amazon's push into fashion relies heavily on athletic apparel.

At least that's what shoppers appear to be buying the most from the retailer's website, according to a new study by Coresight Research (formerly Fung Global Retail & Technology). Nike merchandise leads the way.

Coresight studied 881,269 individually listed fashion products on Amazon.com from Feb. 1 to Feb. 27 to determine where the e-commerce giant has focused its efforts. Nike had 16,764 products listed on the site, the firm found, or roughly 2 percent of all fashion items. Fitness brand Interstate Apparel placed second, with 16,743 products. Gildan Activewear, Adidas and Sport-Tek were all in the top 10.

Further down the list came general apparel merchandise retailers like Ralph Lauren, Aeropostale, Tommy Hilfiger and Hugo Boss.

Many apparel items on Amazon are still sold via third parties. More than 86 percent of the clothing products listed on the site are from third-party sellers, Coresight found.

"We expect to see more first-party listings flow through to the site as Amazon continues to strengthen its relationships with major brands," Coresight founder and CEO Deborah Weinswig said. "The retailer has much room yet to run in private-label apparel."

Of nearly 1 million fashion products, Amazon only features about 830 private-label items, Coresight found. Amazon's women's brand Lark & Ro accounted for 454 of them, while men's brand Goodthreads sold roughly 60 pieces of clothing.

It appears Amazon is focusing on "higher-value categories" with its own brands and other first-party distributors, Weinswig said. For example, fewer third parties are found selling blazers and suits on the website.

"We think the retailer's reliance on third-party sellers underscores its opportunity to grow apparel sales by bringing more inventory in house," Weinswig said. "If Amazon held more first-party clothing inventory, it would mean that more products would be eligible for Prime delivery, boosting the appeal of such items among Prime members."

Amazon has been testing with Nike to sell its merchandise directly on the site. Calvin Klein is another recently added partner.

Walmart and Target, meanwhile, have been making a push with their own private-label apparel lines. Walmart is also preparing to roll out a revamped website later this year, with new fashion verticals. It will partner with Hudson's Bay-owned Lord & Taylor to sell merchandise directly on Walmart.com, threatening Amazon's plans to hook up with other retailers.