By Eva Dou in Beijing and Alistair Barr in San Francisco 

U.S. tech companies are caught in the middle of an escalating battle between China's increasingly active Internet censors and the free-speech activists determined to thwart them.

Activists outside of China say they are disguising Internet traffic banned by Beijing--which includes anything from social networks such as Facebook and Twitter, to Gmail and news websites--by tunneling it encrypted through cloud servers run by major U.S. companies.

These cloud services run by Amazon.com Inc., Microsoft Corp., Akamai Technologies Inc. and others are meant to help businesses improve their website speeds by storing their data on remote servers. But activists say they are using them to get around China's so-called Great Firewall, which could draw the cloud providers unwillingly into the censorship clash.

To stem the flow of prohibited content, authorities would need to block entire servers of these companies, disrupting hundreds of businesses, according to the activists.

"Essentially, it's similar to forcing authoritarian regimes to kill everyone in a protest because they can't tell the real agitators from the bystanders," said Adam Fisk, a Los Angeles-based programmer who in 2013 founded an app called Lantern that is designed to outmaneuver censors. Lantern uses the method along with other groups such as anticensorship activist group Greatfire.org and anonymous communications software provider Tor.

The workarounds are generally implemented without the cloud providers' consent and threaten their operations in China. Verizon Communications Inc.'s cloud service EdgeCast was blocked in November last year, while other cloud providers have cut off service to activists in an effort to avoid getting blocked. The Cyberspace Administration of China didn't respond to requests for comment.

China's Internet censors have strengthened content screening in recent months, creating difficulties for businesses and disrupting more commonly used firewall-circumvention software called virtual private networks, which connect users to the Web through a proxy server overseas. President Xi Jinping has ordered tighter control of online content that may undermine the ruling Communist Party, with bloggers facing jail for spreading what the government says are false rumors.

The cloud-cloaking process used by Lantern depends on what are called content delivery networks, or CDNs, a service offered by the cloud-computing arms of companies like Amazon and Microsoft. The cloud helps businesses' websites run faster by saving copies of its data at multiple locations around the world.

An activist can sign up for a free or paid account with a network and link a blocked website to it. Users behind the firewall download an app or software that routes them to the mirror version of the banned website. Since the data gets encrypted, censors can't see the content.

Reporters Without Borders said last week it used a similar tactic to unblock nine banned political websites in different countries, such as the Tibet Post in China and the Gulf Center for Human Rights in the United Arab Emirates.

"The philosophy is to make it as expensive to block as possible, so that there would be a lot of collateral damage," said Philipp Winter, a Swedish computer scientist who researches censorship technology.

U.S. cloud companies are wary of being seen by Beijing as linked to activists, especially in a worsening climate for foreign tech firms.

CloudFlare, which offers content-delivery network services, said last week it cut off Lantern's use of the service, saying it was unauthorized. "We don't do anything to thwart the content restrictions in China or other countries," said Matthew Prince, chief executive of CloudFlare. "We're a tech company and we comply with the law."

Lantern's Mr. Fisk said he believes people should have access to the unfiltered Internet. "The freedom of access to information is a bedrock of an informed citizenry," he said. Lantern has more users in China than it does in any other country.

China hasn't resorted to widespread blockages of delivery networks because of the potential economic consequences, activists say. China's market for cloud-computing services reached US$1.1 billion in 2014, and is expected to grow by 45% this year, according to research firm IDC. Many of the country's biggest firms rely on such services, as do around 60% of Chinese small- and medium-size companies, according to a survey last year by S&P Consulting.

Many technology companies have come up against ethical dilemmas related to China's Internet firewall. Google Inc. pulled some of its operations out in 2010 over censorship issues. Cisco Systems Inc. was the target of a 2011 lawsuit by free-speech advocates for selling equipment to China that helped censors block websites. Cisco, which won a dismissal of the lawsuit last year, said it sold generic routers to China and wasn't involved in censorship.

Greatfire.org began creating mirror sites in 2013 and now has 10, including a copy of Google, an uncensored version of Chinese microblog website Weibo, and a news website called Boxun that is often critical of the Chinese government, according to a person who identified himself as the spokesman for Greatfire.

The organization uses Amazon and Akamai, among other providers, for its mirrors, according to Greatfire's spokesman. Amazon declined to comment. An Akamai spokesman said the company has no record of Greatfire using Akamai's service, adding that the company's policies forbid customers from "placing illegal content on our platform in any jurisdiction, including China."

Tor, a software for anonymous online communications, has used the workaround after it was initially blocked in China in 2009, said one of the group's founders, Roger Dingledine. Several Tor add-ons were deployed in the past year that disguise Tor traffic as Skype calls, or as data from cloud-service providers like Microsoft Azure or Amazon, and so far they are working in China, he said.

Tor's programmers need to constantly refine their techniques as China's censors are constantly strengthening the firewall, according to Mr. Dingledine. "This is an arms race," he said.

Write to Eva Dou at eva.dou@wsj.com and Alistair Barr at alistair.barr@wsj.com

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