Smartphone maker Samsung has claimed that Apple's FaceTime video calling feature infringes a patent owned by the Korean manufacturer.
The Galaxy smartphone maker has demanded six million dollars (£3.6 million) as a result of the alleged infringement as it began its own case for damages.
The trial between the two technology giants is now into its fourth week, and sees Apple seeking two billion dollars (£1.1 billion) from Samsung over claims that it infringed Apple-filed patents over products.
After three weeks of defence, Samsung has claimed that Apple's FaceTime app, which allows users to make video calls to other iPhone or iPad users via an internet connection, infringes a Samsung patent regarding the compression of video before transmission over a mobile network.
The court heard that the patent was awarded to Samsung in 1994 and the idea was considered "revolutionary" at the time, according to one industry expert. Apple's lawyers claim that the technology described in the patent is now obsolete.
This latest patent war between the two technology giants surrounds a lawsuit brought by Apple over five separate patent infringements, including the "slide to unlock" feature that is prominent on both Apple and Samsung devices.
Both companies have listed 10 or more products from the other that infringes a patent, and it is the latest in a series of lawsuits between the two firms involving the alleged copying of technology in modern smartphones.
Apple was awarded the previous case in 2012, but the damages paid by Samsung were dropped from one billion dollars (£593 million) to around 800 million dollars (£475 million).