Google has sued British Telecom in the latest escalation of an acrimonious war over patents.
The search giant launched the counter lawsuit against BT and Ipanema Technologies in California.
It has also launched a separate lawsuit against BT in a UK court, but the details have not been released.
The Californian suit relates to four patents for conferencing services and quality assurance, which Google argues are being infringed by enterprise services run by BT on its IP network.
The lawsuit was noticed by Reuters, who reported Google was suing BT Conferencing, BT Americas, BT INS and Ipanema Technologies over the patents 5,581,703, 5,701,465, 6,807,166 and 7,460,558.
But Google's action is just the latest battle in a war which has been running between the two companies for more than a year.
In December 2011 BT sued Google over claims that its Android operating system infringed several of its patents.
BT claimed billions in damages from Google, saying that it had patents relating to Google Maps, Google Music, Android Market and location-based advertising.
"BT's constant investment in innovation has seen it develop a large portfolio of patents which are valuable corporate assets," the telecoms company said at the time.
But Google now says that BT was selling patents to third parties, also known as 'patent trolls', to file new lawsuits. Google said it had been forced to defend itself.
It told Techcrunch:
"We have always seen litigation as a last resort, and we work hard to avoid lawsuits. But BT has brought several meritless patent claims against Google and our customers–and they’ve also been arming patent trolls. When faced with these kind of actions, we will defend ourselves."