It seems Google's $750 million acquisition of AdMob, a mobile display advertising company based in California, has not been as smooth as the online giant might have hoped.

In a blog post, Google has revealed that the US Federal Trade Commission has been investigating the buy-out, despite Google's stance that it didn't "see any regulatory issues with this deal, because the rapidly growing mobile advertising space is highly competitive with more than a dozen mobile ad networks".

"Since we announced our plans to acquire AdMob", Google says, "we've been excited about the positive reaction - particularly from advertisers and publishers who have told us that they're enthusiastic about the possibilities for how the combination of AdMob and Google can improve the effectiveness of mobile display advertising".

It seems the FTC is not so enthusiastic about the potential domination of the mobile advertising space that the deal could bring, as Google reveals that following weeks of talks with the regulator, its received "what's called a 'second request', which means that the FTC is asking for more information so that they can continue to review the deal".

Google says it's working "closely" with the FTC, and is "confident" of a decision in its favour.