The startup, heavily backed by Amazon, is known for its Claude chatbot and AI models, and positions itself as focused on safety and responsible development.

Companies based in China -- as well as in other countries including Russia, North Korea and Iran -- are already unable to access Anthropic's commercial services "due to legal, regulatory, and security risks," the company said in a statement dated Friday.

Yet some groups "continue accessing our services in various ways, such as through subsidiaries incorporated in other countries," it said.

"When these entities access our services through subsidiaries, they could use our capabilities to develop applications and services that ultimately serve adversarial military and intelligence services and broader authoritarian objectives," Anthropic warned.

Anthropic -- valued at $183 billion -- said that the change would affect entities more than 50 percent owned, directly or indirectly, by companies in unsupported regions.

The update to the company's terms of service affects such entities "regardless of where they operate," it said.

ChatGPT and other products from US competitor OpenAI are also unavailable within China, spurring the growth of homegrown AI models from Chinese companies such as Alibaba and Baidu.

Some users in China access US generative AI chatbots such as ChatGPT or Claude using VPN services.

- First mover? -

"This is the first time a major US AI company has imposed a formal, public prohibition of this kind," said Nicholas Cook, a lawyer focused on the AI industry with 15 years of experience at international law firms in China.

"The immediate commercial effect may be modest, since US AI providers already face barriers to operating in this market and relevant groups have been self-selecting for their own locally developed AI tech," he told AFP.

But "taking a stance like this will inevitably lead to questions as to whether others will or should take a similar approach."

An Anthropic executive told the Financial Times that the move would have an impact on revenues in the "low hundreds of millions of dollars."

The San Francisco-headquartered company was founded in 2021 by former executives from OpenAI.

It announced this week it had raised $13 billion in its latest funding round, saying it now has more than 300,000 business customers.

The number of accounts on pace to generate more than $100,000 annually is nearly seven times larger than a year ago, Anthropic said Tuesday.

When questioned on Anthropic's announcement at a regular briefing, Chinese foreign ministry spokesman Guo Jiakun said he was not familiar with the situation.

"The Chinese side has consistently opposed the politicization, instrumentalization, and weaponization of science, technology, and economic and trade issues. Such practices are not in the interest of any party," he said.

Assumptions that the US was far ahead of China in the fast-moving AI sector were upended this year when Chinese start-up DeepSeek unveiled a chatbot that matched top American systems for an apparent fraction of the cost.