The screws are tightening ahead of a major Communist Party summit.

By Feliz Solomon
September 25, 2017

China has reportedly blocked most services of the Facebook-owned WhatsApp messaging app, as online censorship increases ahead of a major political summit next month.

Citing communications experts, the New York Times reports that the service, which China had already temporarily tried to curtail in recent months, was seeing disruptions as early as Wednesday, and that by early this week the block was comprehensive.

“This is not the typical technical method in which the Chinese government censors something,” Nadim Kobeissi, a cryptographer at the Paris-based start-up Symbolic Software, told the Times, suggesting that the country’s censors may have invented specialized software to interfere with the app’s encryption technology.

The disruption is seen as a setback for Facebook fb CEO Mark Zuckerberg, who has been pushing to tap back into the Chinese market. WhatsApp was reportedly the only Facebook product still allowed in mainland China since the social media giant was blocked from the country in 2009.

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Several other U.S.-based online services are banned in China, including Google googl , Twitter twtr and most major Western news websites. Many consumers are embracing Chinese online services such as WeChat, which is similar to WhatsApp but easier for the government to monitor, and Weibo, a micro-blogging site similar to Twitter.

While virtual private networks are often used to circumvent government monitors and censors, authorities have also begun restricting VPN providers. In recent years, Chinese authorities have stepped up online censorship and tightened the legal screws on news media, churches, lawyers and activists.

The latest crackdown comes in the lead-up to the Communist Party Congress in Beijing, beginning on Oct. 18. The summit, held once every five years, is expected to reconfirm President Xi Jinping as the party leader and head of state.

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