Yahoo!’s patent battle with Facebook is getting nastier.

In court papers filed on Friday, Yahoo! expanded its six-week-old lawsuit against Facebook to include more allegations of intellectual property theft.

Yahoo! says Facebook’s social network infringes 12 of its internet patents. That’s up from 10 alleged patent violations previously.

Yahoo! also denied allegations that it has been infringing on 10 of Facebook’s patents and accused its rival of engaging in shady behaviour.

Among other things, Yahoo! says Facebook violated an agreement between the companies to notify each other of possible patent infringements before filing a court claim.

The acrimony is spilling out as Facebook Inc is preparing to raise $US5 billion ($4.84 billion) in an initial public offering of stock that is expected to be the richest in Silicon Valley history.

AP