HTC has extended its patent lawsuit against Apple, adding nine patents which it bought from Google last week.
It's added five of the patents to its complaint with theInternational Trade Commission (ITC) and the District Court of Delaware, and has also filed a new complaint in Delaware with the other four.
It alleges patent infringement by Apple's iOS devices and Mac computers in the user interface, over-the-air updates, notifications and application syncing of the iPhone, iPad and iPod touch.
"HTC will continue to protect its patented inventions against infringement from Apple until such infringement stops," says Grace Lei, HTC's general counsel.
By handing over the patents to HTC, Google is sending a clear message of support to Android vendors.
"Google knows that HTC is under tremendous legal pressure from Apple and clearly on the losing track," says patent expert Florian Mueller.
"Another motivation for Google is probably to demonstrate some support to third-party Android device makers even though it's clear those won't be able to compete with a Google-owned Motorola Mobility on a playing field if the deal goes through."
He says that Google's support for HTC makes it more likely that Apple will sue Google directly.
Over the last year, patent litigation has turned into something of a feeding frenzy, and many observers believe the US patent system is in urgent need of reform.
Later today, the US Senate is to vote on amendments that would switch the US from a first-to-invent system to a first-inventor-to-file, bringing it more in line with the rest of the world.