The startup helps web site publishers sort through various advertising networks and choose the cheapest possible ads to run on their sites. Google hasn't confirmed this information, as sources say the $400 million deal with AdMeld is still pending.

This corner of the online advertising field is heating up, as New York-based AdMeld has already received $30 million in venture capital and competes with Rubicon Project and PubMatic. Especially important are the interactive, video and game-based ads all three rivals encourage publishers to buy.

AdMeld helps website hosts display interactive ads in real time, meaning the second someone clicks on a site, various advertisers bid against each other to show the user eye-catching ad videos and games.

If it goes through, AdMeld acquisition would give Google an advantage in display ads, an area where it doesn't yet dominate, despite its omnipresence in the online advertising world.

The Mountain View, Calif.-based company bought Invite Media last year, which helps advertisers purchase ads in real time, so AdMeld's acquisition would cement the demand side of the equation.

Google's reported talks with AdMeld are the latest bout of the search giant's acquisition fever. In March, Google announced plans to buy 50 startups this year, mostly in the mobile sphere but some in the online ad business as well. Last year, the company outbid Apple for mobile advertiser AdMob; Google already owns DoubleClick and AdSense.

But Google's shopping spree could be cut short by U.S. government security, as the Justice Department builds an antitrust case against the ever-expanding company. After Google bought up travel software company ITA, the Justice Department shone a flashlight in the company's eyes by demanding hearings over this and other potentially monopolistic practices.

Google has set aside $500 million to settle if the government ever brings this case to bear.

Google faces watchful eyes not only in the U.S. but also in South Korea, as its Fair Trade Commission is also investigating whether Google skews search results to block out competitors and prevents other search software on Android-based smartphones.

If it can manage to acquire AdMeld without raising too many government eyebrows, Google stands to benefit from the purchase. If not, however, the company may find its new acquisition will cost it much more in antitrust legal fees.