BEIJING—A popular U.S. coding website is enduring an onslaught of Internet traffic meant for China’s most popular search engine, and security experts say the episode likely represents an attempt by China to shut down anticensorship tools.

The attack on San Francisco-based GitHub Inc., a service used by programmers and major tech firms world-wide to develop software, appears to underscore how China’s Internet censors increasingly reach outside the country to clamp down on content they find objectionable.

The Cyberspace Administration of China didn’t respond to a request for comment Sunday.

Security experts said the traffic onslaught—called a distributed denial-of-service attack in Internet circles—directed huge amounts of traffic from overseas users of Chinese search giant Baidu Inc. to GitHub, paralyzing GitHub’s website at times.

Specifically, the traffic was directed to two GitHub pages that linked to copies of websites banned in China, the experts said. One page was run by Greatfire.org, which helps Chinese users circumvent government censorship, while the other linked to a copy of the New York Times ’s Chinese language website.