The online advertising market is becoming a battleground

The Federal Trade Commission (FTC) has requested extra information about the $3.1bn (�1.6bn) deal.

Such requests usually lead to a lengthy anti-competition review.

The FTC's inquiry began after rival Microsoft, which recently bought web ad firm Aquantive, complained the merger could have a monopolistic effect.

The organisation will examine whether the deal could hamper competition in the online advertising market.

Separately, Google is also facing questions about its privacy practices from an EU panel.

'Sufficient concern'

In a filing with the Securities and Exchange Commission, Google said it was "working closely with DoubleClick and will cooperate fully with the FTC's request for additional information and documentary materials".

Earlier it had said it was certain that regulators would approve the deal.

Google agreed in April to buy New York-based Doubleclick, which helps link up advertising agencies, marketers and web site publishers hoping to put ads online.

The friendly takeover offer was the first in a flurry of deals as global technology firms, including Yahoo and Microsoft, have rushed to tighten their grip on the hugely lucrative and fast-expanding online advertising market.

Microsoft and another rival, AT&T, had clearly raised "sufficient concern to merit a second request", said competition lawyer Andrew Klevorn.

"They're just being good cops, checking things out before they let it go forward - if they let it go forward".

The FTC can demand that Google and Doubleclick hand over large numbers of documents related to the deal and then have 30 days to decide whether to oppose the takeover.