Facebook has been caught out paying a top public relations firm to plant negative stories about rival Google in the US media.

Burson-Marsteller, one of the world's largest PR firms, tried to get USA Today and the Washington Post, among others, to write critical stories about Google's privacy policies.

Part of the email exchange between a blogger and Facebook's PR firm.

But a red-faced Facebook has owned up to the smear campaign.

The revelation came to light in leaked emails this week and can only worsen relations between the two technology rivals.