The Federal Trade Commission (FTC) has asked for more information about Google's plans to buy Admob, a company specialising in ads for small screens.
The search engine giant said last month that it hoped to buy the Silicon Valley start-up for $750m (£449m) in stocks.
Google's group product manager Paul Feng wrote in a blog post that the company "doesn't see any regulatory issues" with the proposed deal.
He admitted that the scrutiny meant the deal would not be closed immediately.
It took months for the FTC to approve Google's earlier purchase of another internet advert company, DoubleClick.
"Closer scrutiny has been one consequence of Google's success," added Mr Feng.
Admob was founded four years ago and sells adverts designed to run on mobile phones.
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