Red Hat Linux uses Microsoft's intellectual property and Red Hat customers need to pay up for the privilege, Microsoft boss Steve Ballmer has claimed.

Ballmer made the claim, an echo of earlier remarks aimed at the open source community, during a presentation at a Microsoft event in London last week.

"People who use Red Hat, at least with respect to our intellectual property, in a sense have an obligation eventually to compensate us," Ballmer said.

Red Hat said in response that its customers are protected from liability by its Open Source Assurance Program, which includes "indemnification against claims raised by any holder of software patents," according Red Hat's website.

Red Hat is also a founder of the Open Invention Network, which provides "a patent safe harbour for the Linux environment.”

Ballmer's remarks are "unfortunate," said Jim Zemlin, executive director of the Linux Foundation. If Microsoft really believes Linux violates its patents, it should sue, but that by doing so, it would be suing its own customers because many use both Microsoft and Linux.

"They have yet to actually make a specific allegation that an identified patent is infringed in a specific product. Until they do it, they'll just create more ill will in the development community and among their own customers, since by their own admission, most of them have both Microsoft and Linux running in their datacentres," Zemlin says.

He urged Microsoft to help to reform the software patent system if it supports interoperability with open source. Microsoft entered into an agreement with Linux vendor Novell a year ago to improve interoperability with Windows and Novell's SUSE Linux and protect Novell customers from intellectual property liability.

A Gartner report says Microsoft's strategy is not to file suit but use the infringement claims as leverage to win other agreements like the one with Novell.

"We think the company will attempt to pressure technology providers to come to the table and negotiate an equitable licensing or royalty arrangement in instances where Microsoft can prove its claims of infringement," Gartner said in response to a May Fortune magazine article in which Microsoft claimed open source software violated 235 of its patents.