(Reuters) -A federal judge in San Francisco ruled late Monday that Anthropic's use of books without permission to train its artificial intelligence system was legal under U.S. copyright law.

Siding with tech companies on a pivotal question for the AI industry, U.S. District Judge William Alsup said Anthropic made "fair use" of books by writers Andrea Bartz, Charles Graeber and Kirk Wallace Johnson to train its Claude large language model.

Alsup also said, however, that Anthropic's storage of the authors' books in a "central library" violated their copyrights and was not fair use.

Spokespeople for Anthropic and attorneys for the authors did not immediately respond to requests for comment on the ruling on Tuesday.

The writers sued Anthropic last year, arguing that the company, which is backed by Amazon ( AMZN ) and Alphabet, used pirated versions of their books without permission or compensation to teach Claude to respond to human prompts.

The class action lawsuit is one of several brought by authors, news outlets and other copyright owners against companies including OpenAI, Microsoft ( MSFT ) and Meta Platforms ( META ) over their AI training.

The doctrine of fair use allows the use of copyrighted works without the copyright owner's permission in some circumstances.

Fair use is a key legal defense for the tech companies, and Alsup's decision is the first to address it in the context of generative AI.

AI companies argue their systems make fair use of copyrighted material to create new, transformative content, and that being forced to pay copyright holders for their work could hamstring the burgeoning AI industry.

Anthropic told the court that it made fair use of the books and that U.S. copyright law "not only allows, but encourages" its AI training because it promotes human creativity. The company said its system copied the books to "study Plaintiffs' writing, extract uncopyrightable information from it, and use what it learned to create revolutionary technology."

Copyright owners say that AI companies are unlawfully copying their work to generate competing content that threatens their livelihoods.

Alsup agreed with Anthropic on Monday that its training was "exceedingly transformative."

"Like any reader aspiring to be a writer, Anthropic's LLMs trained upon works not to race ahead and replicate or supplant them - but to turn a hard corner and create something different," Alsup said.