CALIFORNIA, USA: Zynga Inc said it bought four small mobile game companies for an undisclosed sum as the top maker of Facebook games seeks to expand its lineup on smartphones and tablets.

The company's top mobile executive David Ko told Reuters on Wednesday it had acquired German company Gamedoctors in December. Gamedoctors, based in Bielefeld near Hanover, makes the game ZombieSmash.

The company also bought Page44 Studios, which is based in San Francisco, in September. That studio created the "World of Goo" game for Apple's iOS platform.

Zynga acquired HipLogic, another San Francisco-based games company, in August. Ko also confirmed Zynga purchased New York based Astro Ape Studios in August to develop new titles.

Ko declined to comment on the size of the deals.

While Zynga was one of the earliest game makers on the Facebook platform, it lacks that first mover advantage on mobile, where companies such as Walt Disney Co and Electronic Arts Inc have released hit games.

The company's shares have been lagging since it went public in December. The stock fell 6 percent on Wednesday to close at $8.65 per share.