In a strongly worded rebuttal, Amazon.com on Thursday countersued IBM. The companies have charged each other with patent infringement.

The countersuit, filed in the US District Court for the Eastern District of Texas, denies that Amazon.com infringes IBM patents, seeks to invalidate some IBM patents and charges IBM with infringing on some Amazon.com patents.

IBM illegally uses Amazon.com technology in its widely deployed application server, WebSphere as well as in IBM information management services and products, Amazon.com claims. IBM and its customers thus infringe on Amazon.com patents, the web retailer said.

The Amazon.com patents enable a website to recommend products to customers based on their browsing and buying histories.

Amazon.com made the claims in response to a patent infringement suit that IBM filed in October.

In addition to the counterclaims, Amazon.com argues that some of the patents that IBM charges it with infringing are unenforceable. "IBM's broad allegations of infringement amount to a claim that IBM invented the internet," the suit reads. "If IBM's claims are believed, then not only must Amazon.com pay IBM, but everyone conducting electronic commerce over the World Wide Web (indeed, every website and potentially everyone who uses a web browser to surf the web) must pay IBM a toll for the right to do so." The suit calls that idea "nonsense."

Amazon.com also refutes IBM's accusations that it dragged its feet replying to IBM's initial attempts to negotiate over use of the patents. In reality, Amazon.com has already proved that some of IBM's claims are without merit, it says. After IBM first approached Amazon.com in 2002, the US Patent and Trademark Office (USPTO) found that IBM was concealing prior art, which can invalidate a patent. The USPTO then told IBM that the patents were not infringed, were not valid and did not require a licence, Amazon.com said.

Amazon.com also pointed out that IBM waited until 2002, seven years after Amazon.com started up and just as Amazon.com became profitable, to ask for payment for using the patented technology. This shows that "IBM simply wants to siphon off the profits that Amazon.com has only recently earned through the professional risks and human efforts, ingenuity, and investment expended by Amazon.com as it charted a new path in the internet age," the lawsuit reads.

Amazon.com asks the court for compensation for IBM's infringement as well as damages and legal costs.

At the time of IBM's initial filing in October, patent experts observed that such infringement suits are risky because of the potential for judges to rule that a patent is invalid. They also said that the delay in making initial contact – seven years after Amazon.com launched – could work against IBM.