Popular messaging app WhatsApp has suffered serious disruption to its service in China.

According to the Open Observatory of Network Interference (OONI), network measurement data suggests that Chinese internet service providers started blocking access to WhatsApp on Saturday. Public reports on Twitter indicate that WhatsApp, which is owned by Facebook, became inaccessible for some people a week ago.

In China on Tuesday, users reported intermittent service on the messaging platform.

Over the last few months, there have been a number of WhatsApp disruptions in China. WhatsApp declined to comment on its status there.

The most recent move to censor the encrypted messenger comes ahead of next month’s 19th National Congress of the ruling Communist Party. At the sensitive gathering, which takes place once every five years, the government will select leaders and determine policy priorities.

Related: WhatsApp is being targeted by China’s censors, experts say

China regularly tightens its internet restrictions in the lead-up to major Communist Party meetings.

“Typically, in the run up to Party Congresses, we’ve seen blocking, filtering, restrictions on the internet, and that’s what we’ve been seeing in the last couple months,” said Adam Segal, director of the Digital and Cyberspace Policy Program at the Council on Foreign Relations.

The Chinese government runs a huge apparatus of internet filters known as the Great Firewall, which it uses to censor content that it deems harmful.

However, the latest move against WhatsApp is also part of a wider trend of tightening controls and restrictions under President Xi Jinping, Segal said. Although China has reinstated WhatsApp access in the past, it might not lift the block moving forward.

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WhatsApp users on international SIM cards and data plans have not experienced the same disruptions. The restrictions appear to specifically target China-based users.

China’s internet regulator did not immediately respond to a request for comment.

Blocking WhatsApp further chills Facebook CEO Mark Zuckerberg’s longtime efforts to make Facebook services available in China.

Related: China says VPN crackdown aimed at ‘cleaning’ the internet

Big U.S. companies have been shut out of China’s market for years, including Facebook, Instagram, Twitter, and Google. Some people access these services through virtual private networks, or with tools that disguise internet traffic to circumvent censorship. But the Chinese government has been cracking down on VPNs this year.

According to Timothy Heath, senior international defense research analyst at the RAND Corporation, the Chinese government does not like that WhatsApp uses strong encryption.

“The government wants to monitor internet communications, and therefore it’s trying to steer its people to use technology that can be accessed and monitored by the government,” Heath told CNN Tech.

Earlier this month, WeChat, a popular chat service in China, notified users of its policies to comply with government requests for information.

— CNN’s Nanlin Fang contributed to this report