Sega said Friday it has acquired U.S. online game developer Three Rings Design, in an effort to grow outside its traditional video game business overseas.

"With this acquisition, Sega aims to aggressively expand its business in the rapidly expanding online game industry in the European and U.S. regions," the company said in a press release.

The purchase by Sega, known for its Sonic and Yakuza game lines, is the latest in a series of similar acquisitions by Japanese firms looking to break into online and mobile gaming away from home. Earlier this year, mobile game operator Gree purchased the U.S. online game company OpenFeint, and last year social game producer DeNA acquired the mobile game firm ngmoco.

Sega and Three Rings previously collaborated on the Spiral Knights multiplayer online game. Like many online worlds, the game is free to join, but users can purchase power-ups and other items during game play.

Three Rings CEO Daniel James said his company, which is based in San Francisco out of an office designed to look like the submarine in Jules Verne's 20,000 Leagues Under the Sea, will operate as a development studio within Sega. The company is currently working on an online role-playing game based on the Doctor Who franchise.

Sega said it officially completed the deal on Oct. 31. It did not reveal financial details.