Chinese authorities appear to have severely disrupted the WhatsApp messaging app in the latest step to tighten censorship as they prepare for a major Communist Party congress next month.
Users in China have reported widespread disruptions in recent days to the Facebook-owned service, which previously malfunctioned in the country over the summer.
Experts said the problems began on Sunday, but text messaging, voice calls and video calls appeared to be working again on Tuesday, though voice messages and photos were not going through.
WhatsApp provides message encryption technology that likely does not please Chinese authorities, which closely monitor and restrict cyberspace through their "Great Firewall".
China has tightened online policing this year, enacting new rules that require tech companies to store user data inside the country as well as restrictions on what is permissible content.
Websites such as Facebook, Twitter, Pinterest and a slew of foreign media have been blocked for years.
The WhatsApp troubles emerged ahead of the Communist Party congress on October 18, when President Xi Jinping is expected to be given a second five-year term as the party's general secretary.
"It smells like Party congress pre-emptive blocking," said Jason Ng, who researches China's internet at the Citizen Lab at the University of Toronto.
China usually steps up surveillance around major events, Ng said.
While the WeChat messaging app owned by China's Tencent company is more widely used in the country, many WhatsApp users complained about the disruptions.
Many Chinese activists prefer to use Whatsapp because of its end-to-end encryption.
"As we get closer to the Party congress, I think authorities will use more extreme censorship measures. The public knows that WeChat isn't safe," prominent Beijing-based activist Hu Jia told AFP.