Google's bid for a bigger slice of the display advertising market received a boost Friday as the Justice Department gave the green light to its $400 million purchase of online ad company Admeld.

The Justice Department said a review of the transaction by its antitrust division had concluded it was "not likely to substantially lessen competition in the sale of display advertising."

"Although the Antitrust Division concluded that this particular transaction was unlikely to cause consumer harm, the division will continue to be vigilant in the enforcement of the antitrust laws to protect competition in display and other forms of online advertising," the department added in a statement.

The Mountain View, California-based Google has come in for increasing regulatory review in the United States and Europe as it has grown from a scrappy startup into an Internet titan.

Google makes most of its money from advertising tied to Internet search but has been seeking to increase its share of the growing market for display ads, which include rich media, digital video and banners.

Google announced the acquisition of Admeld, which provides an online display advertising platform for publishers, in June. The exact purchase price was not revealed but various reports put it at around $400 million.

The Justice Department said it examined whether the purchase of Admeld "would enable Google to extend its market power in the Internet search industry to online display advertising through anticompetitive means."

It said the investigation determined that Web publishers tend to rely on multiple display advertising platforms, lessening the "risk that the market will tip to a single dominant platform."

Google welcomed the Justice Department's clearance of the deal.

"We'll close the acquisition in the coming days and then start the real work -- building improved products and services that help our publisher partners to make more informed decisions across all their ad space, and to grow their revenues," Google's vice president of display advertising Neal Mohan said.

"The opportunity for major online publishers is huge... and growing," Mohan said in a blog post. "We envisage a much simpler system that enables publishers to manage and sell their ad space -- across desktop, video, mobile, tablets and more."

According to the Interactive Advertising Bureau and PricewaterhouseCoopers, US online advertising revenue rose 15 percent last year to a record $26 billion with display ads accounting for 38 percent of the total.

According to digital marketing firm eMarketer, Facebook is on track to pass Yahoo! in US display advertising revenue this year while Google will also gain market share.

Facebook's share of US online display ad revenue will grow to 17.7 percent this year, up from 12.2 percent last year, while Yahoo!'s share is expected to decline to 13.1 percent in 2011 from 14.4 percent last year, eMarketer said.

Google's share of US display advertising revenue will grow to 9.3 percent this year from 8.6 percent last year, eMarketer said.

Other large Google deals that have drawn scrutiny from the US authorities are its $3.1 billion purchase of online ad firm DoubleClick, its $750 million buy of mobile ad network AdMob and its $700 million acquisition of flight data company ITA Software.

In 2008, Google was forced to drop plans for a joint search advertising partnership with Yahoo! amid opposition from the Justice Department.

Google is also currently cooperating with a US Federal Trade Commission probe into its search and advertising business.

Founded in 2007, Admeld describes its mission on its website as providing publishers with "the cutting edge of advertising technology, enabling them to maximize their revenue and sell their inventory smarter and safer."

Besides its headquarters in New York, the company also has offices in San Francisco, London, Berlin and Toronto.

Admeld's 500 customers around the world include FOX News, Hearst Television, Discovery and The Weather Channel.

Google shares were up 1.22 percent at $621.24 in afternoon trading on Wall Street.