Zynga, the biggest developer of games played on Facebook, has countersued Electronic Arts, alleging unfair competition.

EA, the second-largest US video-game maker, sued Zynga on Aug. 3, claiming Zynga’s “The Ville” video game infringed copyrights for “The Sims Social,” an EA game that runs on social-media site Facebook. Zynga yesterday filed counterclaims in federal court in San Francisco.

Zynga has 274 million monthly users on Facebook while Redwood City, Calif.-based Electronic Arts has 53 million players.

Activision Blizzard Inc. is the largest US video-game maker.