Facebook did not immediately respond to a request for comment.

The government is in the midst of preparing for a sensitive party congress while Chinese censors this week revved up a sprawling effort to scrub all mention of Liu Xiaobo, the Nobel Peace Prize laureate who died Thursday in government custody.

A report this week by the University of Toronto's Citizen Lab detailed how Chinese censors were able to intercept, in real time, images commemorating Liu in private one-on-one chats on We Chat, a feat that hinted at the government's image recognition capabilities.

Signal, another encrypted messaging service, appeared to also have patchy service with significant delays.

China has long blocked Twitter, Facebook and You Tube, with officials arguing that foreign social media services operating beyond their control pose a threat to national security.

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But there was no evidence to suggest that Chinese authorities were decrypting Whats App messages, Kobeissi added.

A Chinese censorship researcher known by his pseudonym Charlie Smith said authorities appeared to be blocking non-text Whats App messages wholesale precisely because they have not been able to selectively block content on the platform like they have with We Chat, which is produced by Shenzhen-based internet giant Tencent and legally bound to cooperate with Chinese security agencies.

Nadim Kobeissi, a cryptography researcher based in Paris who has been investigating the Whats App disruption, said he believed The Great Firewall was only blocking access to Whats App servers that route media between users, while leaving servers that handle text messages untouched.