Rakuten Inc., Japan's biggest online retailer, is aiming to start full-service sites in the U.K. and Germany by the end of this year, part of its effort to challenge Amazon Inc. and eBay Inc. in Europe.

The sites will be built alongside a parallel search for acquisition targets on the continent that could build Rakuten's European footprint, Chief Executive Officer Hiroshi Mikitani said in an interview in Paris. Tokyo-based Rakuten last year bought France's PriceMinister.com for €200 million ($282 million), and paid $250 million for American site Buy.com.

Online retailers are hunting for new territory and expanding into new technologies as growth slows and competition intensifies in their home markets. EBay, the operator of the world's largest online marketplace, may open new local-language sites in Eastern Europe, Latin America and Asia, CEO John Donahoe said in an interview Tuesday.

"There is a simultaneous process, which is looking for possible acquisitions or joint ventures," Mikitani said. "If we can add, we will, but for now we will build our own operations."

Rakuten's flagship shopping portal, Rakuten Ichiba, connects some 22,000 merchants to customers looking for a one-stop shop for products from electronics to pajamas. The company has used its customer base to sell extra services such as credit cards, which generated about $771 million in revenue last year, or 18 percent of total sales.