Facebook has fired back at Yahoo with a countersuit charging that the floundering internet pioneer is violating the social network's patents - and not the other way around.
The move came after Yahoo filed suit against Facebook in a California court last month accusing the social networking giant of infringing on 10 patents.
"From the outset, we said we would defend ourselves vigorously against Yahoo's lawsuit, and today we filed our answer as well as counter-claims against Yahoo! for infringing 10 of Facebook's patents," said Facebook general counsel Ted Ullyot.
"While we are asserting patent claims of our own, we do so in response to Yahoo's short-sighted decision to attack one of its partners and prioritise litigation over innovation."
Yahoo, in a 19-page lawsuit filed in US District Court for the Northern District of California, accused Facebook of infringing on patents in several areas including advertising, privacy and messaging.
In the suit, Yahoo said Facebook's growth to some 850 million users "has been based in large part on Facebook's use of Yahoo's patented technology."
The Sunnyvale, California-based Yahoo! asked the court to order Facebook to halt its alleged patent-infringing activities and to assess unspecified damages.
Yahoo did not respond to requests for comment.
But its lawsuit was filed as Facebook was poised to go public on Wall Street, perhaps as soon as May, with a valuation of up to US$100 billion.
Facebook recently amended paperwork filed with the US Securities and Exchange Commission to warn potential investors that Yahoo's patent lawsuit could deliver a significant blow to its business.
"This litigation is still in its early stages and the final outcome, including our liability, if any, with respect to these claims, is uncertain," Facebook said in the updated SEC filing.
"If an unfavourable outcome were to occur in this litigation, the impact could be material to our business, financial condition, or results of operations."
Patent suits are a frequent occurrence among smartphone and tablet computer makers, and the world's best known brands are ensnared in a complex web of legal claims, but such suits are relatively rare among social media companies.
The countersuit rejected a claim that "without Yahoo's achievements websites such as Facebook would not enjoy repeat visitors or substantial advertising revenue."
Facebook, which is shifting operations to a former Sun Microsystems campus in the Northern California city of Menlo Park, denied violating any valid Yahoo patents.
Facebook went on to accuse Yahoo! of infringing on its patented technology in a broad array of products including online venues for news, games, cars, travel, and photo-sharing service Flickr.
Facebook asked the court to dismiss Yahoo's complaint entirely and make the Internet veteran pay damages as well as the social network's legal costs.