Google is recruiting developers to work in-house on mobile apps for its Android operating system, a report says, as the tech giant continues its challenge to Apple's iOS and the popular devices that run on it.

Benjamin Ling, a Google product-management director, has been supervising an attempt to coax software engineers, user-interface specialists, and product managers into the Google fold, The Wall Street Journal reported, citing unnamed sources.

The Journal added that some current Google workers have shifted their positions at the company to join the app project, which will be spread across Google's global offices and cover everything from Angry Birds-like games to Foursquare-like check-in apps. The resulting apps will probably be free, with ads included to generate revenue, the Journal said.

The mobile battle has been heating up for Google and cross-Valley rival Apple. Google has been breathing down Apple's neck in the smartphone market, and some say tablets such as Motorola's Xoom, which run on Google's "Honeycomb" OS for tablets, might well give the wildly popular iPad a run for the money.

Apple, meanwhile, is finally offering up the iPhone on Verizon, and has a healthy lead when it comes to apps. More than 350,000 paid and free iPhone and iPad apps are available in Apple's App Store, and more than 10 billion have already been downloaded since the store's launch in July 2008. It's a general industry rule of thumb that the more apps available for a given device, the more potential buyers that device can attract.

Google's Android Market currently features less than 130,000 apps, but its stable of offerings reportedly grew more than 500 percent last year, compared with Apple's App Store, which saw offerings rise by about 110 percent. Developers, it seems, are warming up to Android.

And apart from company rivalries and struggles between devices, the market for mobile apps is nothing to sneeze at. Gartner recently predicted that such apps will generate $15 billion in revenue this year, with downloads more than doubling, to 17.2 billion from 2010's 8.2 billion.