Apple has claimed HTC is breaking patent agreements for its own mobile handsets.
The latest filing from Apple at the Delaware District Court claims HTC was breaking four of its patents for the use of software in its smartphones.
However, it is not the first row between the two rival firms.
Back in March, Apple filed a complaint, both to the Delaware court and the International Trade Commission (ITC), claiming that HTC broke 10 patents around touch screen capabilities on the Nexus One.
HTC responded with its own counter-suit, and it claimed Apple had infringed five HTC patents, calling for a ban on the sales of iPods, iPhones and iPads until the case was resolved.
Apple is still embroiled in a case with Nokia and one with Kodak, but the former has been called to a halt whilst Steve Jobs' boys clear up the other cases at their door.
Strung Robertson, a technology lawyer for international law firm Pinsent Masons, told IT PRO recently: "It is a bit of an arms race. What often happens is that large companies build patent portfolios to use for offensive purposes and also defensive purposes."
He added: "What you get is one company threatening another with patent litigation and the company that is threatened might be able to fire back and say 'Well, if you are going to raise this issue, we will bring up this other patent that we have that we think your other technology might be infringing'."
