San Francisco/Beijing, Sep 26: After blocking videos, voice chats and images on WhatsApp, China has now blocked texting on Facebook-owned popular mobile messaging service -- sending out a stern message to CEO Mark Zuckerberg who has been trying his best to re-enter the country. According to a CNN report late on Monday, the Open Observatory of Network Interference (OONI), a global observation network for detecting censorship, surveillance and traffic manipulation, suggested that Chinese internet service providers started blocking access to WhatsApp. Public reports on Twitter indicated that WhatsApp became inaccessible for some people on September 19. Over the last few months, there were a number of WhatsApp disruptions in China. "The blocking of WhatsApp text messages suggests that China's censors may have developed specialised software to interfere with such messages, which rely on an encryption technology that is used by few services other than WhatsApp," The New York Times reported. The move came ahead of a key Communist Party Congress on October 18 at which President Xi Jinping will begin his second five-year term as head of the ruling party. However, WhatsApp was yet to make an official announcement on the development. China has already blocked access to a number of internet companies, including Facebook (since 2009), Instagram, Twitter and Google. Some people still access these services through virtual private networks (VPN), or with tools that disguise internet traffic to circumvent censorship. But the Chinese government has launched a crack down on VPNs this year. According to Timothy Heath, senior international defence 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. Facebook was banned in China in 2009, followed by its photo-sharing app Instagram in 2014. Facebook's founder Mark Zuckerberg has made a big point of meeting with Chinese politicians, studying Communist Party propaganda, Mandarin and even speaking it in public. In a bid to explore the high-potential Chinese online market, Facebook recently authorised the release of a new app here that does not carry its name. China's internet censorship has left big players like Facebook, Apple and Google out of the huge Chinese market with an audience of more than 700 million internet users. Last month, in a crackdown on internet services by the government, Apple had removed all major VPN apps from the App Store in China.