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 had previously malfunctioned in the country over the summer.
Experts said the problems began on Sunday, although text messages, voice calls and video calls appeared to be working again on Tuesday, though voice messages and photos were not going through.
China has tightened online policing this year, enacting new rules that require tech companies to store user data inside the country as well as imposing restrictions on what is permissible content.
Chinese cyberspace regulators said Monday they slapped "maximum" fines on major Chinese tech firms Baidu and Tencent for allowing the publication of pornographic, violent and other sorts of banned material on their social media platforms.
"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.