Facebook on Thursday denied waging an alleged smear campaign against Google after it admitted having secretly hired a top U.S. PR firm to plant controversial stories on Google to media outlets.

Facebook insisted in a statement that "no smear campaign was authorized or intended," saying it hired Burson-Marsteller (B-M) to make media outlets or analysts independently verify the privacy concerns on Google's social networking practice.

The social network site got embarrassed after B-M issued a statement saying that the Facebook assignment "should have been declined" and Facebook has requested its name be withheld.

The alleged smear campaign gained wide attention when USA Today broke a story earlier this week, saying that B-M has been pitching negative stories about Google to top-tiered U.S. media outlets on behalf of an unnamed client.

USA Today reported that B-M sent two high-profile former media figures to urge the newspaper run a cover story and editorials about how Social Circle, a Google Gmail feature, ostensibly tramples the privacy of millions of Americans and violates federal fair trade rules.

USA Today said it decided the claims were exaggerated and B-M declined the newspaper's requests for comment.

"We have seen this e-mail reportedly sent by a representative of the PR firm Burson-Marsteller," Chris Gaither, Google's senior manager of global communications and public affairs, was quoted by the newspaper as saying.

"We're not going to comment further. Our focus is on delighting people with great products," he added.

Meanwhile, former U.S. Federal Trade Commission researcher and blogger Christopher Soghoian posted on-line a full e-mail text from B-M dated May 3, in which the PR firm offered to ghost write an op-ed column critical of Google and get it published in The Washington Post, Politico, The Hill, Roll Call and The Huffington Post.

Fingers have been pointed at Apple and Microsoft when B-M at first refused to disclose the client. The Daily Beast, an American news reporting and opinion website merging with Newsweek last November, broke the news that the mysterious company was Facebook.

According to The Daily Beast, a Facebook spokesman confirmed the hiring of B-M Wednesday night, citing two reasons. The social network said it believes Google is doing something in social networking that raise privacy concerns and Google attempts to use Facebook data in its own social-networking service.

Google's Social Circle is a tool to help remind Gmail users of the contacts they regularly email or chat with, so-called direct connections. It also privately sends Gmail users the names of " secondary connections," a list of the people each direct connection happens to be following publicly on-line.

The "Facebook assignment" reflects the fierce competition between the two Silicon Valley giants on on-line advertising. Both companies want to gather information on users, their friends and what they like to sell targeted advertising. It is reported that Google has been using tracking programs and algorithms to connect more members from Facebook to Gmail users.

The pitching practice also questions the ethics of the PR industry, as PR professional should decline a client when the request is improper, some analysts said.

Facebook is under media fire for the practice. In an interview with technology website Betabeat, Christopher Soghoian, source of the B-M pitching, said that "this was an attempt by one large company to stab a dagger in the back of a competitor."

TechCrunch, a well-known IT news and analysis website, noted that Facebook engaged in "cowardly behavior in battle" and it is hard to trust the company on other things.