Zynga Inc is hiring two key developers of cocos2d, a popular game engine that powers top grossing iPhone apps such as Tap Pet Hotel and Tap Zoo in a move that will help the social games company boost its presence on smartphone and tablets.

Zynga, the privately held social games company, has expanded rapidly through small acquisitions at the rate of about one a month in the past year, as it seeks to dominate the games market and expand internationally.

Under the agreement, the developers, Ricardo Quesada from Argentina and Rolando Abarca from Chile, will work at Zynga's San Francisco office. The developers used to work at Sapus Media, which was acquired in the deal. No financial terms were disclosed. The employees start working at Zynga on Monday.

Zynga said it would not be acquiring cocos2d, which is a free game engine that developers use to make games for Apple devices such as the iPhone, the iPad and iPod touch.

Zynga's Chief Technology Officer, Cadir Lee, said in an interview that Zynga already uses the game engine in its FarmVille game on the iPhone. Having the developers, who were key in developing the free platform, will help Zynga master the technology, Lee said.

We will be paying them to continue to develop the platform, as well as work on tools for Zynga, to make sure we can leverage the (platform) in the best way possible, Lee said.

Lee added that Zynga plans to have a diverse selection of game engines to choose from when making its games and will not just stick to one development platform.

Over time, we're going to do more and more with a variety of game engines, whether it's 3D, HTML, 2D -- we want to make sure we have a complete portfolio, Lee said.

Zynga is the top games publisher on Facebook and its main revenue generator comes from selling virtual items such as tractors and weapons in games that users can play on their PCs, but it has taken steps to reduce its reliance on Facebook.

The company is creating more mobile games that users can play on smartphones and tablets, said David Ko, the executive who heads Zynga's mobile business.

We want to make sure we invest in mobile as the company recognizes its importance going forward, Ko said in an interview.

Ko said the acquisition of the developers Quesada and Abarca, as well as the April 27 acquisition of United Kingdom-based Wonderland Software, a nine-person studio, will give Zynga a stronger foothold in the mobile market, where it competes with companies such as Electronic Arts Inc and Rovio, the maker of Angry Birds.

Ko said the acquisition of the Wonderland studio in April should reap another fun game for our players and that Zynga should be releasing more games soon.

Where we are in the industry for mobile, we're still in the infancy. The industry is just getting started, Ko added.

(Reporting by Liana B. Baker; editing by Andre Grenon)